Yahoo Widget Engine (formerly Konfabulator)

Yahoo's site has plenty of other free widgets, including RSS readers, games, an iTunes remote controller, and even a tuner for listening to Sirius satellite radio on your PC (but you'll need a subscription for that). You can even find a widget based on the TV show Lost, in which you can enter the numbers "4 8 15 16 23 42" every 108 minutes, just like the characters did down in the hatch.

--Preston Gralla