You can acomplish this with Audacity - but it's not as straight forward as the other solutios proposed (WireTap, AudioHijack, etc)...
You'll need both Audacity and
Soundflower.... channel your computer's audio to soundflower, make Soundflower Audacity's input source, and hit record. Also, once you have Soundlower as a valid output source, you can record the computer's audio with any recording software - Garageband (which is way better than Audacity), Quicktime (which can also record audio).
But if you plan to do this regularly, and/or expect a good quality recording, I second the
WireTap Studio suggestion, since it lets you decide which audio sources you record - safari's, skype, system audio, etc - and isolate it from the rest.
A very inexpensive suggestion is
Boom - although audio channeling is not its primary function, it presents itself as an internal audio input/output source, so it can be easily selectable in audacity or Garageband, without the hassle of Soundflower. It's a great little app.
Hope this helped. Cheers!