I wish to share some of my favorite Horror Manga here are the links enjoy:

Uzumaki - The spiral
This follows a young girl trapped in a mysterious cycle that doesn't truly unfold till the very end.

Flesh Coloured Horror
A collection of short horror stories.

Emerging
There is an outbreak of something new that causes those infected to suffer a horrible death.

Mimi no kaidan - Mimi's Ghost Stories
A set of horror stories

Yin Yang Road
Another collection of short stories

Anamorphosis
A group of people is invited to stay on a haunted movie set with a chance at winning a large sum
This also includes short stories

more to come...