DEAD Tube
If you don't mind high levels of brutality, excessive nudity, and other psychological shit, I recommend trying out this manga. It is, by far, the BEST HORROR/SLASHER manga out there (for me). The mangaka is very familiar and experienced in drawing the "ecchi" scenes, and does so while displaying a brutally shocking amount of gore for horror fanatics. Gotta keep an open mind before reading this one tho :P
Dolls Fall
Some creepy yuri shit right here. I love it. (gave me some nightmares tho.)
Gekkan Shoujo Nozaki-kun
A good laugh, guaranteed :-)
Hanako to Guuwa no Tera
SPLENDID! I thought they're just multiple stories but it ALL ADDS UP in the end. Definitely one of the BEST, storyline was done so well. Rec this one to those interested in Japanese urban legends and horror stories with a bit of romance ^^ The ending makes me want to read again. It's the same author from "Future Diary" so why not? :)
Btooom!