Well saying something that is good is always prone to biases. To start it off, try checking One Manga's list of Mature mangas. Then try to read Seinen too. Different manga categories can also be considered mature, that is, upon your context.
Let's see... I think Shamo is a bit good. I don't really know, I'm reading lots of manga, too many to remember at once.
Yakuza Girl. It's a shounen thats more mature than most seinen.
I Love You!
Last edited by ImotoChan; 02-20-2009 at 07:07 AM. Reason: Merged
I've just finished reading Voyeur. It's about people who watches other people having sexy sexy time. It's good :B
Hmmm...I don't normally read Seinen but I happened to have picked up two books from the series "The Gentlemen's Alliance Cross" at the library. It's not bad, I might get out some more when I have the time. Here's a link to what it's about: http://en.wikipedia.org/wiki/Shinshi_Doumei_Cross :3
I highly recommend Dorohedoro which is becoming my favourite seinen manga.
It's funny, thrilling, violent, unpredictable, full of good ideas, has great characters/universe...
Gotta admit the wiki's description kinda sucks. You got to read at least 2 volumes to make your mind.
You can find downloads at manga underground.