You can’t record vocals, but the Lightsnake cord ($70) from Soundtech makes it possible to record instrumental tracks on a computer without a sound card. You plug one end of the cord into your instrument and the other into a USB port, and the green lights come on to tell you that sound is being recorded. You can use it with Apple Garageband (Free*), Sony Acid Music Studio ($69), or any other digital recording software you like. (via OhGizmo!)
Perhaps this progressive simplification of the home studio is the reason there are so many crappy bands with recorded tracks on MySpace.
*Garageband is part of iLife, which comes bundled with new Apple computers.