Although I should probably come up with something more squamous than “plethora.”
Anyway: I think that I’ve previously mentioned Trail of Cthulhu, which is Ken Hite’s exceptionally good sourcebook for 1930’s-style Cthulhu Mythos roleplaying (the default time period is actually the 1920s, which is of course a completely different style from a roleplaying standard) for the GUMSHOE system, which is an attempt to create a roleplaying game that is actually designed to accommodate mysteries and investigation. It’s the same game engine that’s used by Robin Laws’ The Esoterrorists, which more of a modern occult-horror investigation/repression game.
Good gaming stuff, in other words – and Steve Jackson Games is selling supplements for both Trail of Cthulhu and The Esoterrorists as part of their PDF publishing service. Significant savings over the dead-tree version, instantly accessible, good stuff with which to get your geek on – a nice deal all around. Check them out