The Lightsnake XLR Intelligent Microphone Cable can take any XLR device, from mics to keyboards, and connect them to your PC or Mac through USB. It requires no drivers, and is Garageband compatible out of the box.
The cable comes packaged with a mic and vocal training software. Think of it as a Karaoke Revolution for the sober nongamer. I tried matching a C, and succeeded marvelously as an (obviously) highly accurate line graph charted my performance.
For any home composer or podcaster, this is a must buy at $89.99.