Yashiro Tenshuu killed 4 people, and is now on death row. However, a secret government organization is willing to take him off death row if he allows himself to become a test subject to an experiment that requires a killer's intent.

From Mangascreener:

The only common feature running through these stories is a grown-up protagonist somehow coming face-to-face with his childhood past, in a testament to the loss of youth, the disillusionment of growing old, and nostalgia for better times.

In the Adachi pantheon, this is a rare diametric view of his eternal theme of "youth," in this case youth as seen through the lens of the past. It's a powerful statement from Adachi that might even be an acknowledgment of his steadily-creeping age.

What is impressive about these stories is not the plots or characters themselves, but the absolutely perfect way he tells them, another facet of experience that comes with age. Most of the stories may require multiple readings to fully appreciate the airtightness of the plot and visual motifs."

Etou Kai may look like a squeaky-clean kid, but the Greater Kanto Narcotics Control Division's new investigator hides a violent alter ego and a dangerous past.

With his stoic partner Kurabayashi Hal, Kai is assigned to track down and stop the distribution of Dragon Speed, a dangerous new drug. But Ryuugen, the syndicate trafficking the drug, is almost impossible to infiltrate. Add street violence and a jurisdiction war with the Meguro Police into the mix, and it looks like Kai's in for some very long days at the office.

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…

Picking up directly after the events of 20th Century Boys, it seems like the world is finally out of danger, but the mystery of "Friend" still exists. As the world is threatened again, Kenji must search his memories for any clue about "Friend."

Note: 20th Century Boys and 21st Century Boys' combined storyline won the Grand Prize at the 37th Japan Cartoonist Awards on May 9, 2008.

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

There is a mysterious group of people who inflict cruel and fatal games on people for their own amusement. Tatsuya and Misa are out on a date in an amusement park when they wander into one of these horrific competitions on the tallest roller coaster on earth. The seats fall out of their 12-person roller coaster car, and they are told that the game will end when all but one person has fallen to their death. Will the passengers be able to find a way to escape the competition? Will there be any retribution against the orchestrators of this sick crime?

Satou Atsushi isn't neglected by his family so much as ignored completely. Left to fend for himself, Atsushi fails to learn human speech and instead learns "cat language" and "Dog language" from the family pets. When he's eventually socialized with other children, his speech is stunted and his social graces leave something to be desired. But on his first day of nursery school, he meets a gloomy blonde with the same last name who seems to be a safe enough choice in a first friend. After developing a rather sizeable fixation with Satou Manato, he finds out that there his first friend is seriously ill and has a twin; Shuuto.

Years later, two other friends begin school and find that there are two Satous in their class too. Two Satous who, at first glance, could easily be mistaken for twins, but there's a bit more to things than it first appears...

A graveyard in contemporary Israel has an unlikely visitor. The elderly gentleman from Japan, a former news correspondent, lays a bouquet of flowers at the tomb of one Adolf Kamil. For he remembers the tale of three Adolfs: Kamil, a Jew who grew up in Kobe, Japan, the son of a baker; Kaufmann, only child of a German consul stationed at that port city and his Japanese wife; and the Fuhrer with whom the Far Eastern nation made common cause.

Won the 10th Kodansha Manga Award for general manga in 1986 (shared with What's Michael?).

Abara is set in a dystopian world, filled with large structures. The story revolves around creatures known as Gaunas, those who can shape bone like armor and weaponry around themselves by transforming. The transformation begins around the spine and then shapes layered armor.

The story focuses on one such Gauna known as Kudou Denji, though he uses Itou Denji as a fake name early on.

Abara means "ribs" in Japanese.

Volume 2 contains Digimortal
A mercenary in cyber-armor battles a repressive cyborg regime.

Description from Viz: The story A, A' raises a troubling question: How would you feel if your lover died - and was replaced by a clone? Could you recreate your relationship with someone who was physically exactly the same but didn't share any memories of your life together? In 4/4, teenaged Mori flunks out of an ESP training program. But when he meets Trill, a beautiful Unicorn, his latent abilities begin to blossom... In X+Y, Hagio uses science fiction to explore questions of gender and sexual identity. Time has passed, and once again, Mori finds himself inexorably attracted to a member of the unicorn race - but this unicorn is male... and so is Mori. X+Y won Seiun Award in manga category in 1985

Undercover investigator Matsuzawa Tomoya has a blood-stained past and hidden abilities. His new suspect, Kiryuu Yuuki, is one who looks exactly the same as the bloody man who has been in Matsuzawa Tomoya's mind for the past 15 years.

From Serenus-Dreamers:

1. Ageha
Yuuri and Miu had always been best friends. They tried hard together to become fashion models. However, Miu got to debut first, and their relationship started to break down. Surging dark feelings and jealousy... what is there beyond Yuuri’s agony…?!

2. Eternal Rainbow

3. Kiss Me, Baby

The metropolitan police have a special squad of negotiators for talking down violent criminals in hostage situations and so forth. The two top negotiators have great knowledge of human psychology and experience in dealing with criminals, but they are missing something important.

They are searching for someone who is capable of agape love-- unconditional and selfless love for all people. Such a person would be able to develop rapport easily with criminals, and therefore be a natural negotiator. Instead of finding a person like this on the police force, they find her in the form of the teenage girl Haruka. Will they be able to persuade her to become a negotiator for the police?

Taken from Solaris-SVU:
Ikuru is a boy who doesn't have long to live. Growing up questioning the purpose of his life and the reason for his impending death, he's constantly confused and lonely. After learning that he doesn't have much time left, he requests that a special "girl" called a AGH-RMS (Artificial Genes Human Regenerated for Mental Support) be given to him. These people are genetically made so that they help heal and ease the mental burdens of the terminally ill. This manga is cute, sad, and ultimately a story about life, death, and humanity.

1) Yurusarenai Otoko

2) Aishiatteru Futari

3) Seigi no Mikata no Koi no Yukue (Love & Battle)
Seigo, who grew up in a village, moved to Tokyo to find a job and to fight crime (apparently he has some strange superpowers), since not much happens in his village. One day his childhood friend Touma visits him and challenges him to remind him of his mission. At least that's Touma's claim...

4) Kanoko's Story

Note: Chapters 1-3 are shounen ai stories while chapter 4 is shoujo.

From Viz:
What does it take to find your true inner self? Zen's memory has been wiped, and he can't remember if he's a killer or a hero. And a lot of people will do anything they can to keep it that way.