Wired has an article today on science songs written by professors and other groups. Here are sites with full mp3's: Craig Growther, Food Safe, Jim Webb, and Science-Groove.
And here are the sites with samples: Edutunes, Sing-Smart, Joan Maute, Kathleen Carroll, Monty Harper, Professor Boggs, Science Explosion, Science in Song, J.P.Taylor, InYour Backyard, Teacher and the Rockbots, and Why Does the Sun Shine? (though I prefer the TMBG version).
Bear in mind that these are all horrible (but educational!).