My husband ran across Drug Digest's Check Interactions site tonight, and i felt it was too good of a resource to not share. Not only does it include traditional pharmaceuticals, it also includes information on more natural supplements and such.
Of course, don't forget what you already know and take what you read with a grain of salt...it's not going to be a *complete* guide. It does do a nice job of providing basic information on possible interactions and it also gives you info on how well documented the interactions are. It also lists possible food & alcohol interactions.