From Project Kurai Ryu:
Young Kai has been passionately idolizing Ginta ever since his heroic battle in the War Games of six years ago. Since then, ARMs have become a common thing, Guardians especially! Kai himself doesn't have one though, and dreams big of being able to get an ARM like Ginta's! When his caretaker, the old ARM-smith that took him in after his parents' deaths, sends him out on an errand, he is attacked by a bandit from "chess"! In his escape attempt, he finds more than he bargained for when he comes across Babbo! But, now with Babbo's awakening, the fake ARMs are going out of control! Is it Kai's turn to save MARHeaven following in his hero's footsteps?

Akira Kongou is looking to take down the "23 district project" which involves "banchou" from 23 districts of Tokyo fighting for the right to control Japan. Without meaning to, he becomes a participant with the alias Kongou Banchou and must take down other banchou while searching for the leaders of the project.

A one-shot manga. It’s an action/comedy story about a woman named Tyria who is a maid in disguise, but in reality she is sort of a counter-terrorism operative. It's up to her to stop a hostile takeover by pirates on an airship using only cooking items, like a frying pan, cutting knives and even a fork as apart of her deadly arsenal.

A young boy is saved by a mysterious gunfighter who uses magical bullets. She is the greatest magician in the world but disappears shortly after saving him. Having fallen in love with her, he trains to become a gun fighter and ten years later, his search comes to an end but his adventure has just begun. Kurohime is a buxom witch with the power to control magical guns. She once made the mistake of challenging the gods and was cursed for her foolishness! Transformed into a little girl named Himeko, she can only regain her former body and powers when she falls in love. But finding love in a world fraught with evil is no easy task.

From JMS:
Based on the Sega Saturn game of the same name. The story follows the most troublesome class of the Magical School on the island of Yen and their adventure to understand each other and protect the island and school from the Vile Tribe.

In a vast world, Emma, a girl with something lurking inside her, meets the magician Shion!!! What is the "Magico" ritual that will bind them together?! Even if they risk their lives to tread a path of thorns...!!

Plucked out of his small country village, Robin is headed on a long and dangerous journey with two guys he must depend on - a possible cheat named Ted Viper and a ladies' man called Sydney Rush.

This is a cowboy shoujo manga set in the Wild West. Miriam is an orphan. Her father was a drunken gambler; she only had one dress and they moved from place to place, until one day her father died and Grace took her in. The series follows Miriam's life, growing up, and falling in love with her life-long friend, the skillful sharpshooter Douglas.

First of the Miriam series (3 volumes), followed by sequels Jikan wo Tomete Matteite (3 volumes) and Sore nari ni Romantic! (1 volume). Seven volumes total.

My Milky Way Railroad is a oneshot sequel to Hoshi no Harmony.

Thirty years ago a series of meteorites rained down on Earth, destroying much of civilization. Decades later, pockets of humanity are beginning to reemerge, but many of the children who have been born after the crisis have been mutated by radiation from the meteorites. They are called the Meteorite Breed.

Tooi, a fourteen years old kid that eats a lot, is one of the Meteorite Breed. But he is a rare one...

A collection of one-shots:

Vol 1:
Magician
Fairy Tale
Cocona
Plue's Adventures Pt. II

Vol 2:
Bad Boys Song
Magic Party
Christmas Hearts
Fighting Group Mixture

From Neptise Scans:

The story follows Shintaro, the Ganma Army's #1, after he gets stranded on the wacky Papuwa Island.

From Anime New Network: Zushio was to be king and promised his sister that he would become king (on penalty of death). Unfortunately for him, his empire was overthrown and now he's a wanted man. A girl has the misfortune of running into Prince Zushio and becomes tangled in his plight. Adding to her misfortune, Zushio is not quite right in the head, but hey, at least he seems to be immortal.

The tale of Monster Soul takes place in Elfenland, which is inhabited by both humans and monsters. The main characters are a group of monsters called the Black Airs. The Black Airs is recorded as the strongest group of the monster forces in the great war between the humans and monsters.

The story takes place in a fictional world full of monsters and adventurers called Monster Hunters, who, as their name implies, hunts these monsters. The plot revolves around a young Monster Hunter, Shiki, whom was taken in as an apprentice to a Monster Hunter named Gurelli as a child. A few years after Gurelli's death, which was due to a gunpowder incident, he comes back to his master's residence, Akamaaya Town, to join the guild there. In that guild, he meets a girl called Irie and after a series of events, he finds that she is the daughter of his master. From this point onwards, they form a party to find the legendary Miogaruna, which was Gurelli's life-long ambition.

From: Easy Going Scans

Mahiro is the prince of the Mismarca Kingdom, he won't study nor learn sword skills, he is a lazy prince who only plays until late night. His Imperial Bodyguard Pariel would always scold him for his behaviour. And towards that Mismarca Kingdom an elite squad from the Demonic Kingdom Granmarsenal is aproaching. The Leader of the squad is Lunas, a warrior princess, the third Princess of Granmarsenal. All of Mismarca's vassals voiced their will to fight and bring all-out war, although there is a man who said cowardly "Why don't we have a talk with them?". What kind of fate awaits the Mismarca Kingdom and its useless Prince, Mahiro?

From Adachi's Universe:
We start with our main protagonist, Shichimi, bidding farewell as he readies for his journey into Edo (on our Earth, Edo was the old name of Tokyo before it became the capital of Japan). His mother has just died, only leaving him a walnut and a request for him to go to his half-siblings. Shichimi bids farewell to a sternly old man, who is a firefighter. We can tell this from the symbol on his cloak. This symbol is also on Shichimi's cloak. Upon reaching the town of Edo, Shichimi meets a mysterious man, who ridicules the peace of the world as an illusion.

Lakate is a man with the face of a wolf due to a curse. While traveling to find a cure for his state and the reincarnation of a woman who helped him long ago, he meets a lost young girl, Kukaru. This is where his real travel begins...

Quoted from 4dawgz (the scanlator)-
"So there's a magazine called Gelatin where various manga artists are given some coloring crayons and then told to draw whatever they want in full color. The plot for this is about a guy and a girl who go on a mission to retrieve a pair of shorts from an island... aaaaand that's it. All you'll find is a nice slice-of-life story and lovely colored pages."

Nanami Sekai gets scammed out of every penny he has, left only with a pair of boxers and a 3,000,000 yen debt. He then meets a girl, Ritori, who offers him a deal that seems too good to be true: she will take care of his debt if he will be her captain and help her with her job.

[Note]
It seems that this manga has been discontinued because the author became incapacitated during serialization and was not able to keep 2 series running.

One day, cute little Tarepanda decides to go on an adventure. But after a few bad turns, he realizes he misses home. Will he find his way back? Or will he stay lost forever?

This little one-shot is simple, charming and cute. If you like sweet slice-of-life / adventure stories, do yourself a favour and go read it!