I was trying to help my daughter grab a sound clip off a Windows game. I did some Googling, and found Wavepad. Bottom line: Wavepad makes it reasonably easy to grab whatever audio is playing through your soundcard (or whatever output device you have) and record it.

The only caveat is this: While Wavepad will ostensibly save your recorded sound in your choice of formats, some of them don't appear to be compatible with other players. Wavepad-generated AAC, for instance, surprisingly wouldn't play in iTunes. For compatibility's sake your best bet is to save in WAV format and then convert it (if necessary) using another program such as iTunes.

Wavepad is currently completely free. As they say on their site, their hope is you'll like the tool so much that you're willing to pay for some add-ons they also make.