My very favorite mystery reference site is Stop, You're Killing Me!. (And it's been nominated for an Anthony! Yay!) If you haven't been using it, you should definitely check it out.
The backbone of the site is a huge index of mystery novels - or rather two of them, one alphabetical by author and one by character name. I have found this invaluable, particularly when I'm trying to make sure I read a series in order. (Non-series novels are covered as well.) I'm sure it's not completely comprehensive, but I can't recall ever trying to look up an author and not being able to find him or her.
And as if that weren't enough, the site has several other great indices, including the Location Index, Job Index, and Historical Index. I often use those three when I'm in the mood for a certain type of book. There's also information on award, read-alike recommendations, and more. Go check it out!