Detroit Metal City is about a death metal band that has gained immense popularity in the indie scene.
The main character is a push-over, Negishi Souichi, who is trying to make a living in the music world, but he didn't imagine in his wildest dreams that it would be in such a band. Read and see what hilarious situations he gets himself into.

From J-Wiki:
Takashi is reunited with a childhood friend he has not seen in 13 years named Miriko. Miriko, while thought to be missing, became the queen of an alien planet and leader of a strange army corp. To keep a childhood promise, Miriko sets out to take over the world with Takashi. However, Takashi has no desire to do this, so Takashi must stop Miriko's wild behaviors.

From: Easy Going Scans

General Tokugawa was given 13 sacred swords to bring peace to Japan. However just when Tokugawa started a new dynasty, he was assassinated by his 12 commanders, aka masters of the 12 sacred swords. The last one, Crimson Firebird, was given to Dohran, Tokugawa's naive son . Many years have passed and now, under the cover of a travelling performer, Dohran is seeking revenge and also to bring back the smile in the faces of people.

Japan in the future has a massive over-population problem. Scientists have traced the root of this problem to one particular individual, Junta Monamari. According to their records, Junta was a "Mega-Playboy" who had sired children with over a hundred women. Each one of his sons had inherited the Mega-Playboy gene too, so the problem grew exponentially from thereon. Enter Karin Aoi, a "DNA Operator". Karin is sent back to the past with a mission: to alter Junta's DNA and prevent him from ever becoming the Mega-Playboy, thereby eliminating the world's overpopulation problem. Junta's DNA can only be altered by shooting him with a special bullet called DCM which contains the chemical for suppressing the Mega-Playboy DNA. In return, Karin gets a vast sum of money... enough to make all her dreams come true. Will Junta become the playboy he is destined to be, or will his better self prevail? But Karin is falling for the real Junta too... and what of the sweet and kindly Ami who has secretly loved Junta all her life? What will happen to future Japan?

A certain boy decides to summon a demon to help him with the search of his beloved...

Sengoku Akira and his classmates are travelling on a plane when it crashes onto an island inhabited by species thought to have been extinct. The situation becomes increasingly desperate as some of Akira's classmates are eaten alive, while some kill others in desperation. Now Akira and a few other survivors are trying to survive what seems to be a horrible nightmare.

Roa Coatl is on a mission. He must find the descendant of Quetzalcoatl to make her his wife and continue his great race of dragons for 'that day'. However, with no knowledge about love and having no interest in women, will he be able to accomplish his task in the human world?

An RPG-ish gag manga about a half-dragon-half-human girl whose love for a popular idol/dragon-slayer leads her on a quest to find a potion that will make her fully human. Which of course leads her and her friends in a quest to defeat the terrible Demon Lord Azatodeth.

To cure his persistent virginity, Yuuto Mutsura joins his friend Naota in a trip to Aqua Eden, the "City-by-the-Sea", where brothels are allowed. Before he has a chance to become man, he is separated from his friend and accidentally turned into a vampire! Now forbidden to leave the city and forced to participate in a Public Morals task force with several beauties, how will Yuuto handle this new life?

A planet-threatening space disease, the "demon flu", has infected an inhabitant of Earth, and a space doctor is needed! The Space Medical Bureau has chosen Dr. Rurru for the job, but -- unfortunately for Minao Hitoharu, the infected patient -- she has a tendency to mishear and misspeak (with a mind towards ecchi) to comical effect.

Features Illya as a main character where she's living an alternate life as a school girl. However, she is chosen by the Magical Stick Ruby and is involved in a quarrel between Rin (Kaleido Ruby) and Luvia (Kaleido Sapphire).

Just before entering the university, Yanagiba Masahiko's father died in a car accident. His mother died when he was four, which makes him now really an orphan. But even before the death of his father, he didn't have a family life, as his father was always busy or abroad.

When his mother's younger brother and his wife decide to invite him to live with their family, his life takes a turn. But it is not going to be the normal family life he is imagining for himself. What is the "terrible" secret that led his parents to break the bonds with his aunt and uncle? What could be the true nature of his cousin, Shion? And last but not least, will this "terrible" secret eventually get Yanagiba, too?

Onizaki Teppei is a high-school freshman who just wants to live a plain student life, but his world is rudely shaken by a mysterious girl, Mitogawa Enma, who declares her intent to find a husband at the school!

Sousuke is the class president, and is a stickler for the rules. Lately, he's had lots of problems. First of all, he's got to deal with his cute, but unruly, childhood friend Miroku. Secondly, they have a new transfer student, Enma, who claims to be from hell. Enma refuses to follow any rules but his own. Lastly, there's the giant, mysterious claw marks that have been appearing all over school buildings. With all this trouble going on, will Sousuke follow the rules or learn that sometimes there are more important things?

From ryColaa:
Our hero, who doesn't know how to swim, joins his school swimming club and begins to learn from their star swimmer who loves swimming more than anything. Good thing she's cute and nice to boot!

From Tokyopop:
Aliens have landed on Earth and are now a normal part of society. These beautiful beings have been given the name "DearS" and are trusted and welcomed by most humans. In order for the "DearS" to learn Earth's customs, they are sent to random high schools to "home-stay." When Takeya helps a DearS in his school, she calls him "Master." Thus begins the humorous life of Takeya and his sexy alien follower, Ren, who tries to figure out the wacky customs of this place called Earth!

From Tokyopop:
Hideki's just like every other guy in the 22nd century. He just wants a good job, a good car, and a sexy robot girlfriend to call his own. Until he lands a job, he'll never be able to afford his own 'Persocom' companion. Hideki's luck changes when he discovers Chi--an adorable but seemingly stupid Persocom--tied up in a pile of trash. His first robot companion turns out to be a lot more responsibility than he expected, and she gets him into quite a few embarrassing situations. It's 'boy-meets-girl' for the cyber age.

From Wikipedia:

Shimeru Futaba is a normal high schooler living a normal life, active in his school's wrestling club and slowly getting closer to his awkward love interest, Misaki. This fails to last, as he discovers his family's hereditary genetic defect that becomes active at adolescence. Although it will eventually become controllable, excitement and stress now make Futaba switch sex. Hilarity ensues.

From Village Idiot:
What will happen to an ordinary human teacher who falls into a youkai school, with his only ally(?) a sexy but air-headed nekomata? A slight ecchi and heartwarming read brought to you by the author of “Sore wa Totsuzen, Unmei no Aite ga”

A short story about a cat demon and a teacher.

Loner high school student Moribe Kyouya thought he was lucky to have the beautiful and mysterious new transfer student Baroque talk to him. The next thing he knows, he receives a love letter calling him to the roof, but the confession isn't exactly what he expected...