From Tokyopop:

Three years after the popular video game The World was shut down, Tokio Kuryuu cannot wait for The World R:X to come out. Unfortunately for him, he forgets to reserve a copy and finds himself with no access to the game. That is, until a mysterious new transfer student forces him into the game and makes him her servant.

G.U.+ follows the player Haseo, a powerful PKKer known as "The Terror of Death". Haseo is obsessed with an infamous PKer known only as "Tri-Edge" who attacked a close friend of his sometime in the past. The manga details Haseo's search for Tri-Edge, and his eventual inclusion in a mysterious group known as "G.U."


Back story of the ps2 game (same title)
In 2015 a fire at the main CC Corp. building resulted in the loss of a lot of data, and The World was forced to shut down. By splicing in data from what would have been another game, CC Corp was able to rebuild the game system. The next year, "The World R:2" was released, but many old players objected because they were unable to bring over their old characters. However, many new users were drawn in by the new style, and over 12 million copies were sold. Though not a match for the previous version's 20 million users, it became the top-selling netgame on the market." As a result of the new world, four major clans (Kestrel, Lunar Tree, Canard, and Raven) came to power. The clans actions shape 'The World', and most players end up taking one side or another during conflicts.

From Hidoi~! Translators:
Another tender love story which fails to run smoothly from Kawachi Yukari!
This is a one volume "novel" type manga.
Rui will be a model, and Tougo will be a painter - the two young childhood friends dream about their future. When Rui's father dies when she is 17, she goes to Tokyo, to fulfill her dreams - and meets a handsome make-up artist and...!

A collection of one-shots:

1. 18 Diary -- Uchiyama Ayako has had a one-sided crush on classmate and good friend Kitamura Tooru since they started high school, but his sights are set on another girl. During their last summer together as classmates, she agrees to teach him to swim, but his sole motivation seems to be to impress that girl...

2. 17's Seisho -- Mizutani, depressed that her parents are filing for divorce, drinks alone at a bar when Sakuma her playboy classmate finds her and hits on her. They end up having a one night stand, but that's when all the trouble begins...

3. Tadashii Hoshi no Miagekata -- Kurahashi has been taking care of her little brother Takumu since her mother died. She has her reservations, however, since she can never seem to get the inward Takumu to open up to her. One day, however, Takumu shows signs of emotion when he sees her classmate Hokuto dance at the park...

From ShoujoMagic & Nekohana: A collection of short stories revolving around a girl's first time! Story 1: A Boy's Suggestion, by SHINJO Mayu Kana is on a mission of love. How pathetic would it be if a senior like her completed high school without ever having had a boyfriend? That's when Takuya, a freshman with a reputation as someone you don't want to mess with, confidently asks Kana to go out with him. She has misgivings about him being younger than herself, but can she be adult enough to overlook it? Story 2: Virgin Beast, by MINAMI Kanan See Ren-ai Shijou Shugi, volume 2 side story. Story 3: Let's Go Be Daring!!!, by AYUKAWA Mio Yoshino has a crush on Kanou and is too shy to tell him, but if she doesn't act quickly some other girl might. Yoshino's friend tells her how to becoming daring: Don't wear any underwear! This secret technique certainly catches Kanou's attention when a brisk breeze blows Yoshino's skirt up, and... Yoshino needs to come up with a good lie, fast! Story 4: When I'm with You, I Won't Be Home 'til Morning!, by KOUSAKA Yuuka After two months and three days of dating, high schoolers Eimi and Hiroto are finally ready to have sex. Eimi is nervous, but Hiroto seems okay. She heard that he's a real lady killer and his ex-girlfriend was his college-age tutor. Can Eimi -- a virgin -- possibly measure up to the kind of experience Hiroto has had? What will it mean for their relationship if she can't...? Story 5: The Awakening of Love, by SAKURAI Miya High school girl Asuka has a boyfriend who's 23 years old, Hiroyuki. When people comment that older guys must be better in the sack, Asuka confirms it -- but she doesn't honestly know... because Asuka is still a virgin. Unable to stand it any more, she gives it up to Hiroyuki (who didn't know she was a virgin), but instead of bringing them closer together, it drives them further apart...? Story 6: Smooch Smooch Smooch, by MASUZAKI Yoshino Tomo, age 16, has a crush on the university student Yuuji. They even live together. Problem: Yuuji is her cousin! He's never had a girlfriend, but that's because Tomo harrasses him in a series of various "attack" styles. As her latest attack, designed to make Yuuji jealous, Tomo and her friend Samata pretend to have a relationship when they're in front of him. But when Samata becomes serious about Tomo, Yuuji knows he has to make a decision -- fast. Story 7: Sweet Desires, by SHIGANO Iori Maya caught her boyfriend cheating... Since then she has lost faith in men. But will this change with a sudden kiss, outside a bar... from a kouhai??!!

Our main char falls with a big bang for the new beautiful transfer student Natsuki. While he keeps it in himself, he finds out that Natsuki is quite the rebel who does what she wants, she’s strong enough to break his mean senpai’s arm and send him to the hospital, and she’s the one who just broke his bass guitar. He still follows her around and finds out that she’s quite nice but tries to distance herself from others. The reason is that she’s getting married as soon as she graduates. This is also the reason why no one really punishes her for doing the stuff she does in school. Still our main char falls more for her and it seems like the love is mutual? The hindrances in this story at the end of first chapter are the marriage and the mean senpai who falls for Natsuki after hearing her sing in an instrument shop.

From ShoujoMagic:
It's Ruri's 17th birthday today. And also the day that her relationship with Jyunya begins. However, was it destiny that their love is going to be a tragic one? What will happen to them now, with their parents against their relationship...!?

These are stories that deal with being an underage mother and how the characters dealt with the situation.

by Forbidden Garden:
a series of oneshots:

We are merely 17 years old and in love. And then I suddenly got pregnant, although he proposed to me straight away, my parents were against this, but I don't want to get an abortion, I want to be a mommy. No one can help us... which path is there for us to take?

Little Life Growing Inside - Falling in love, and then getting pregnant, is that a sin? Just because we're still in high school?

Spanning the Lost Life - Getting an abortion during the age of 15, and then being betrayed by the lover. I won't love anyone ever again, definitely...

I Will Become a Strong Mother - I'm a single mother, I have a crush on someone, is having this type of feeling a burden?

Believing the Feelings of Love

A collection of short oneshots: 1) 17 Sai Natsu Aki connfessed one year ago to Yoh, but he was going out with someone at the time. So, when she finds out they broke up; what she does might be good at first. But in the end will she be happy with Yoh? Or will something go horribly wrong? 2) 17 Sai Fuyu Yui loves Maki, (childhood friend), but does Maki love her? During winter break she goes to her aunt's (where Maki lives) and confesses her love. Does Maki love her too? Or is she going to end up with a broken heart? 3) 17 Sai Haru One sentence is all I've ever wanted to hear from her. That's all. I love her. But I can't, I'm not allowed. She's my brother's fiance. Even so, I'll do anything to make her mine. If I hurt her, bring her pain, well, as long as I can be with her. I don't care if she hates me. 4) 17 Sai Aki Nao is in love with her Sempai. His brother confesses to her, and she uses him to get to the guy she likes. Will it work? Will she get her beloved Sempai?

Haruhiko and Kenichi are 17 year old high school delinquents who are inseperable best friends. They've a reputation around Shibuya for being guys you don't want to mess with. Lately, Haruhiko has been fustrated, getting into more fights than usual, insulting women, and ditching Kenichi. Haruhiko can't admit his problem and Kenichi is at a loss for what to do, and soon things spiral out of control. Can their friendship survive? Or will it change forever?

This manga is about a couple of 17 year old guys and is set in Tokyo’s Shibuya expensive neighborhood. They have been doing everything together ever since middle school, where they became best friends. But Haruhiko cannot stand female company anymore and begins to insult them at the place they are used to going. No need to tell that they take it pretty badly and tend to get a tiny bit furious. Kenichi tries to calm things down though. The fact is that Haruhiko has something bothering himself and he cannot let Kenichi knowing about that… (from Daily Manga)

This manga includes two oneshots:
"Tight Rope Walker"
A story involving a hair stylist

Ever since he was little, Akira wanted to be a rugby player. But since he lived on a tiny island with no other children, it was very difficult for him to learn how to play. In hopes of becoming useful to a rugby team, he practiced the one thing he could do by himself, kicking. Now Akira is heading to high school, and he's going to try to make his dreams of playing rugby and having friends come true!

From: Easy Going Scans

If you could find out a little something about the future, what would you do? Get rich off of the stock market? Use the media to become famous as a prophet? Or…

1-3) 37°C no Boyfriend
Wakaba got locked up in an elevator with a completely strange guy. Moreover, they happened to share a kiss. He's someone she'd never meet again... or so Wakaba thought, but it turned out that that guy, Chitose, is the boyfriend of Nao, her friend!
Nao introduced a surprised Wakaba to her upperclassman in her club, Takaomi, but no matter what, Wakaba can't forget about Chitose...
- Awakened Memories

4) Tsukiakari no Himitsu

5) Haru ni Naru Made Matte
(Also included in Houkago, Kimi to Koi o Shite.)

From Noctis Op.92:
What exactly is the difference between a robot and a human?

In a dystopian future in which robots are becoming increasingly commonplace, a young boy named Hogu lives in near poverty with his grandmother and sister. Though he innately possesses a kind soul, he is ostracized by his classmates and frequently bullied. At the beginning of the story, we find Hogu, friendless and alone - until he meets a robot girl...and our story begins.

1-2) Can't Kiss for 24 Hours
A math teacher who has an intense relationship with the problem student, Kikusui Kiyoya. Even though they were a couple in love, one day, Sensei suddenly wants to break up...?

3) If I Didn't Have You
Fujisawa Yuu just moved to a new town. On the first day of school he's warned to stay away from Nanatsuki Haruki. The word around school is that Nanatsuki who is actually 20 years old will sleep with anyone, but only at school. The mystery is why he only sleeps with people at school and why hasn't he graduated yet?

Humanity, having faced extinction at the end of the 20th century, would not have entered the new millennium if it weren't for them. In 1969, during their youth, they created a symbol. In 1997, as the coming disaster slowly starts to unfold, that symbol returns. This is the story of a group of boys who try to save the world.

Won the Kodansha Manga Award in the category "Best General Manga" in 2001.
Won the Seiun Award for Best Comic in 2008.

From Anime News Network:

A collection of short stories that look to mankind's exploration of space, and examine the possibilities that we could face when we decide to take those first fateful steps into the unknown.

A collection of stories:

1-3. 2 Shuukan no Adventure

4. Himeko-chan

5. His Left Eye
A story detailing an encounter between Gerry and Eugene from Barairo no Hoo no Koro, as adults.

6. Belgian Waffle
Andrew, from Barairo no Hoo no Koro, takes a stroll with his old father, bringing about some surprising discoveries, aside from the fact that Belgian Waffles can be sweet.

Timeline (from Oriolidae & Sparviero):

01 Barairo no Hoo no Koro v01 ch01-05
02 J no Subete v01 ch01-05
03 2 Shuukan no Adventure v01 ch05-06
04 J no Subete v02-03
05 Barairo no Hoo no Koro v01 Extra

From Chibi Manga:

5 years ago, Shin was Yumika's private tutor. Now this Shin-kun comes to Yumika's high school as a teacher trainee! But their love from 5 years ago will most probably start to bloom again?!

From Midnight Scans:

Yumika’s first love was her tutor Shin. As she hope to meet Shin once again, which really happens, how will Yumika proceed on to confess to Shin about her feelings…

From Beautiful Soup:

"1999" is an angsty and widely varied series about a bunch of teenagers. One of them is the son of a drug lord and so as you can imagine, all sorts of shit goes down, surrounded by a backdrop of convoluted and fluctuating interpersonal relationships.

This is a plot heavy manga with some mildly-explicit sexual activity.

MAL:
This plot takes a new twist to Romeo and Juliet's forbidden love. It's July 7th, 1999 in Shanghai and there are two gangs fighting for control. Only in the ceasefire zone do people live in peace. In a chance meeting Xiaoxue, who is part of the Toulon's gang, and Dawu, from the Xianglongji's gang meet by chance and without knowing they are enemies. In the heat of the moment Dawu takes him to a place he has the keys to in hopes Xiaoxue will join him, but is turned down. With a simple kiss he leaves Dawu not knowing they will meet again, but in very different circumstances