SongTapper. Tap out the rhythm of a song with your spacebar and it'll tell you what the song is. Probably no help if you sing "Where da hood at" to the tune of "Karma Chameleon,"1 but for the rest of us, who occasionally ask, "What the hell is this song in my head!? You know, it goes da da da daaaaaaa de bop bop la la la..." this is a great tool.
1. I love that ad.