Unearthed Arcana, for D&D 3.5, for some alternate rules that might jazz things up or help you tune the game to what your group wants. (I really like the minor racial levels, and the rules on complex skill checks could be plunked down as well. For a magic game, you could use their rules to switch over to mana-based instead of slots-per-day based magic. And it's got CoC d20's sanity rules in there, which would work great in a d20 Modern game with a horror edge.)
Call of Cthulhu d20, for the aforementioned sanity rules as well as some good rules on plotting and adventure design.
(These are a bit out there, but the obvious good ones have already been mentioned by folks above, so I'm just firing in the oddball ideas.)