If you liked the Oyasumi Punpun manga, the Anime-Planet community thinks you'd like:
The world is never quite the same once we grow up. Villains and robots that once ran rampant in our imaginations are reduced to drawings in Manga, and our lives are distilled until there is little left but the daily grind; so it has been for Kenji. After a childhood of dreams he now runs his late father's liquor store and is raising his sister's child. The memories and friends from his early years bring him some happiness, but they become tainted as a string of murders find connections to his past. Kenji and his friends must now fight to save the future from their past and unravel the mystery of "Friend."
Though not terribly similar plot-wise (20th Century Boys is an epic political thriller, whereas Oyasumi Punpun is about one guy growing up), they have their share of common elements. Both prominantly feature a bizarre suspicious cult (of Friendship or Good Vibrations), cover a fairly long timeline, and have similar elementary-school era arcs (in which a group of boys- and sometimes one girl, run around the city looking for porno magazines and breaking into abandoned "haunted" buildings, among other things).
At any rate, these are two of the best seinen manga available, so if you're into that demographic, I heartily encourage you to check both out.
It's hard to pin down, but I'd say Baka and Gogh and the beginning volumes of Oyasumi Punpun have a similar sort of... soul(?). Both are really great dramas, with a similar style of humor, and would probably appeal to the same audience.
Both manga are about school children and the hardships - either caused by situations at home or at school - that one can go through while in adolescence. Both have very dark undertones and characters that can be pretty multi-faceted.
Somehow, the main character in these bildingsromans manage to be pathetic and lovable at the same time. Similarly, I'm never sure if reading these manga makes me happy or incredibly depressed. I do know, however, that if you liked the adolescent drama of one, you will definitely like the other.
Susamu Nakoshi, a man disillusioned with life, meets Manabu Ito, a young medical student bored with life. To relieve his boredom Ito offers Nakoshi 700,000 yen to undergo a trepanning to see if it will unlock his sixth sense, and Nakoshi eventually agrees. However, much to his dismay and chagrin it actually succeeds, and now Nakoshi must contend with seeing homunculi, the metaphysical and symbolic representations of a person's psychological disorder. He now helps people, usually against his own will and often theirs as well, with resolving their psychological problems.
If you liked the ennui, psychology, and type of character interaction in either of these seinen manga, you should check out the other. Both are fairly bleak, and occasionally depressing, but have similar atmospheres, and will probably appeal to the same audience.