From Tokyopop: Mikako, our over-zealous heroine, has a sister-complex so strong she won't let just any man wed her sister. On an undercover investigation to learn more about the newest bachelor, she ends up moving in with him! Plus the guy is a highschool kid just like her! Mikako's relationship with him starts to grow. What'll happen between Mikako and him?!

I love math. Just after the high-school entrance ceremony, I meet a beautiful girl: Milka. Milka is a mathematical genius. She shows me many math and many elegant solutions. Milka and I spend a long time discussing math in the school library.

One year later, I meet another mathematical girl: Tetra. Tetra is one year younger than me, and asks me to teach her math. While I teach her, she begins to understand math and gradually to love its elegance.

The third series is about logic puzzles, Peano arithmetic, epsilon-delta, Cantor's diagonal argument, Hilbert's program and Gödel's Incompleteness Theorems.

The story revolves around Tsunashi, the child of royalty who was left behind in the lost kingdom of Karan. He is expected by those around him to awaken his great powers that sleep within him in order to revive the kingdom — but rather than fighting, Tsunan enjoys playing in the world of books. However, he must become serious soon, as a new army is threatening to invade Karan.

From MangaHelpers:
Yasaka Tokiwa lives in an unusual time, and according to his dreams, it's about to become even stranger. He's been having a dream of a woman called Ri'in ever since the robot, who he calls Makina, came to his home. He and Makina attend the same school, where their other classmates include a secret ninja (Kusanagi Kanata) and a sorcerer (Yata Haruka).
When Tokiwa, the robot, the ninja, and the sorcerer rush to rescue the student council president from a demon, the boys find that they've all been having the same dream! What is going to happen to their world?

This is a manga with several oneshots.

Story 1: Pure Love Strip
Story 2: Beautiful Beast
Story 3: Morning Waking Kiss

Story 4: Daisy by AMANE Yuko

Story 5: Lustful Darling
Story 6: Love Princess and W Prince
Story 7: Yokujyou Nyan Nyan (Alluring Nyan Nyan)

Who do you think lives next door? A college student?
A secretary? A foodie? A quiet sort?
Or a killer?

Marriage is in shambles. But still their pride stops them from seeking each other. Thats when both drift out to get the taste of some 'Drugcandy' (adultery). A tale of two people exploring out of their relationship.

Fuyune hates Christmas because it only brings loneliness. But will this Christmas be different?

Jang-Mi and Taebong's long-time friendship is put to the test after they both sleep together for one night. Is this love or just sex? Read H-mate ( Hメイト, H메이트) by Turtle Feet now!!

Maybe you'll like: 
+ Brawling Go Manga
+ Skill of Lure
+ Sweet Guy
+ H Campus
+ Yahalue
+ A World That I Rule

From Kodansha:

Class troublemaker Ryu Yamada is already having a bad day when he stumbles down a staircase along with star student Urara Shiraishi. When he wakes up, he realizes they have switched bodies—and that Ryu has the power to trade places with anyone just by kissing them! After figuring out the workings behind this new and amazing ability, Ryu and Urara take full advantage of the situation to improve their lives. But with such an oddly amazing power, just how long will Ryu and Urara be able to keep their secret under wraps?

Yuki Anno just turned into a high school student, she's very nervous and what's worse, she is in a different class from her best friend Jiu. But she resolves to make new friends. Upon meeting Jiu's cousin, Yuki, who has the same name as her, will she be able to open up and show people who she really is?

N/A

From Yaoi Is Life:

Saichi and Kotetsu are drawing manga as a team while living in extreme poverty. At loss for an idea, both of them decided to write a “real BL” based on themselves! They rise to be a star in the BL world, but…?!

"The other short stories within explore a schoolboy crush that turns into something more, an antagonistic relationship between a wealthy heir and his crush, and the love life of Usaki, the editor of Saichi and Kotetsu’s stories" (June).

Mizuki gets confessed to by a boy in front of a special Mirror. Legend has it, that if you confess to someone you like in front of that mirror your wish comes true.
There is a slight Problem in this Equation: Mizuki is just a very feminine looking boy.
Also it seems that he caught the Attention of the Norse Trickster God Loki while glancing into the Mirror. Loki decides out of boredom to mess with Mizuki and turns him into a real Girl.

From MangaHelpers:

Shou lives out in the country. He has a big crush on his childhood friend Sora, but is afraid that she thinks of him only as a little brother. One day, Sora announces that she was abducted by aliens. Later that day, Shou confesses his feelings for her and she dies shortly afterward. Shou is still in shock from her loss, when an alien girl named Lila comes down from the sky. She has a disk that contains a brain scan of Sora, and she can run Sora's personality in her own body. She also wants to have a child with him. Will Shou be able to cope with all of these sudden changes, and will he be able to keep Sora with him after all?

Meguro Mito has always been made fun of for her chubby appearance. Her sole happiness is discussing her favorite role-play game with her two best friends. One day, while she is being bullied, a handsome classmate saves her. Is this the beginning of her first love?

A story about a long-necked girl's love life:
Kanoi Natsuki and her childhood friend Itsuki visit the same school and there seems to be more between them besides friendship. As a Rokurokubi, her quirk of stretching her neck is normal for her and her surroundings, but it's not always her choice to stretch it and although she has feelings for Itsuki, she can't tell him what she feels. Ultimately, she has to go through misunderstandings and shame.
Itsuki, meanwhile, manages to make any awkward situations dissolve or maybe not?

From Sweet Indulgent:

Hana is a normal teenager who goes to school normally, that is until she gets run over by a car... or did she? Suddenly, she finds herself in a weird place known as the "Red Light District"... in EDO PERIOD?! Trying to find a way to get back to her own time, she somehow gets mistaken for a prostitute and has to escape. Not expecting to be saved by a gentleman, she suddenly ends up in a geisha house.. but then she is bought by the same guy who helped her earlier?!

Mori, a cheerful playboy in his sixth year of college, has just fallen in love with a high school girl named Hoshi. From that starting point, Mizu no Iro Gin no Tsuki portrays the relationships among Mori's friends and acquaintances.

Vol 1 contains work originally published in 1998-2000, which was previously collected into a 2000 tankoubon entitled Mizu to Gin. Vol 2 contains work published in 2005-2006.