Here are some suggestions for Horror
Ravenloft campaign setting 3.0 or Ravenloft PH contains madness, horror, and taint of evil rules plus some neat expansions on some classic horror templates, alignment and direct communication with the gods are always hidden.
Denizens of dread is a monster book for ravenloft with great monsters and lots of templates.
Skip the ravenloft heroes and villains books, they are pretty poor.
Bodies and Souls is a pdf with 20 possession templates.
Look in the modern srd arcana and menace manual sections for new cool monsters, many appropriate to scifi or horror.
Call of Cthulhu d20 is a complete on its own supernatural modern horror game with alternate sanity blasting magic and "Things man was not meant to know".
Rokugan Campaign setting, an oriental adventures setting with an area of "taint" and lots of creatures lost to taint.
Mystic Eye Games has a bunch of D&D adventure collections with horror themes as well as two horror themed monster books. These are mostly both in print and available as pdfs as well.