Since you've read Mail, definetely give Kurosagi Corpse Delivery Service a read. If you didn't know previously, both are illustrated by Housui Yamazaki.
Aaaand.... Mushishi. It's a historical supernatural manga, with some tense atmosphere, and some good bits of horror too.

That reminds me:
ever see the anime movie Appleseed? The 2004 version, great movie also check out the new one just released Appleseed Ex Machina (comes out in US in 3 days woots).

I can't remember what it's called or where I saw it but I remember the first chapter or so was about a kid who is normally very gentle but he gets knocked out by some karate guy and then he turns into a berserker and becomes really strong, he still gets beaten by the karate guy but just by a little. During the fight there's a commentary from a gym teacher or something about the history of berserkers or something like that. If anyone knows what this is or where I can find it I would greatly appreciate it.

K.O. Sen?

SeeYa

Gorath

Thanks for all the suggestions! =D

Does the walker choose the path, or the path the walker?

I'd try Domu.A quick read (only six chapters). It's a dark seinen about an old man with esp.
Tokko is a good seinen/supernatural

Fairy Cube Shoujo that my brother will read. Evil fairies taking over the world and stealing bodies.

You rated Hikaru no Go pretty high so maybe Shion no Ou about shogi with a mystery thrown in.

The Hour of the Mice

"Tu es responsable pour toujours de ce que tu as approvoise." Le Renard en "The Little Prince" Antoine de St. Exupery.

Thanks for all the suggestions! =D I've read a few of them(but didn't have them on my list), but I still got some outta your list that I haven't read.

Does the walker choose the path, or the path the walker?