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.)

3, Street of Mysteries explores the fantastical-poetic vein that has forged Mizuki's reputation in Japan. The stories contained in this series hold in common the obsessive ideas of immortality and of life after death. Skillfully blending a range of black humor, drama, and questions of morality, Mizuki shows once more his profound understanding of the human soul.

(paraphrased from Astiberri)

About a play-girl goddess from heaven who is stripped of all her 'goddess' powers and reborn as a human. The story progresses as her incarnated character learns to live, love and find what true happiness is all about.

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.

Thief Gintonic seeks the mysterious treasures known as "The Three Hearts". One night a mysterious woman falls from the sky claiming to be an escaped Princess from a sacked castle. As fate would have it, she has one of The Hearts. But is she really who she says she is? And will Gintonic let her live to fulfill her mission?

In this story, our heroine, Shinobu, is a female ninja in training. The only trouble is, she seems more interested in hanging out with her friend Kaede, and the fact that her ninja Master is a round yellow thing that pretends to be hawk isn’t helping.

There’s one thing I understand when I became an adult.There’s nothing special about boyfriends, sex and kisses. but if I think that, will I be able to find love?

From Lililicious:

In "24, 25," the main character reminisces about a girl she knew in high school.

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.

From Tokyopop:

Akira - who later appears in the CLAMP School Detectives manga series - is a dapper, debonair young thief-for-hire who becomes the object of 5-year-old Utako's affections.

From Boys On Boys Love:

1-2. 2-dome no Koi no Hajimari wa
After being harshly dumped by his lover, Kousaka Yuuki, a shop manager at a restaurant, swore that he would never love again. One day, he meets his ex again when he unexpectedly shows up at Kousaka's shop. It was then completely exposed to Matsuoka, a kouhai at work, that he dated a guy. Kousaka was shaken that his past was known to Matsuoka, and worried that he would look down upon him, but Matsuoka forcibly kisses him and says "I love you" to Kousaka...!?

3. Gekiai Honey
Sasaki Ryoutarou, a part-timer at a restaurant leading an ordinary life, suddenly met his old classmate, Mio. Mio was once a gloomy guy ignored by all, but is now a cheerful, completely changed guy. He claims to have been searching for Ryoutarou to be his lover because he couldn't forget about him, even after being rejected a long time ago. Mio changed his style because he had heard that Ryoutarou's type is a cheerful person. Ryoutarou refuses to go out with him, to which Mio responds: "go out with me for a month, if you don't fall in love with me then I'll go back"...

4-5. Honey Trap
You is in love with his best friend Shinji. Afraid of ruining his friendship with Shinji, he doesn't confess, and breaks off all contact with him. Three years later, running away from some stalker, he enters a building and asks for help. However, it turns out that the man living there is his ex-friend Shiniji, who is now a detective...

But what will happen when Shinji suddenly asks You to be his lover?

6. Bokura no Koi no Hajimari wa

- Extras
Is a small continuation about the first story.

N/A

Ikumi and Kunitachi are finally hitting it off steadily. However, trouble starts once again during Valentine's Day when Ikumi received a box of chocolates containing glass and became the victim several other pranks. In the end, when the person revealed himself to Ikumi, the past was unfolded.

Unconsciously jealous over what he's afraid to know, Ikumi threw a fit and said something cruel towards Kunitachi. Unable to take it, Kunitachi raped Ikumi...

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

1) Water Breath
An injury destroyed Olympic hopeful Ryuji Hayashida's promising swimming career. He was transferred to a university where he believed no one knew who he was, but he was wrong. There was someone who recognized him the moment their gazes met at the university's swimming club.

2) Pension
A tempestuous tale of a rock band and the consequences of their enigmatic guitarist's return. It didn't take long to realize he has a keen interest in their lead singer.

3) Dance
Shachi never got over his feelings for his classmate Michio who jokingly danced with him at school. Years after their school graduation he discovered what Michio did for a living. The discovery inspired him to sell his father's car for one night with Michio, which turned into a different kind of dance.

4) Longing
Shy and timid high school student Sakaki has always watched his classmate Ryoji from afar, but he wasn't as invisible as he believed himself to be.

The student council president is gaga over the vice president, but her feelings may or may not be returned. In the meantime, it's up to the serious and by-the-book vice president to keep the flaky (but surprisingly effective) president on track.