If you play electric guitar or bass, and don't want to fork over the cash for an amp, here's a solution:
- Visit your friendly neighborhood electronics store and buy a 1/4" -> 1/8" audio jack adapter.
- Plug your guitar or bass' amp cable into the adapter and plug the adapter into the audio-in jack on your Mac.
- Open Audio MIDI Setup in the Utilities folder. Click the "Audio Devices" tab (I'm on 10.2, so this might be different on Panther). From the Input Source, select "Audio In", then check the "Play Through" checkbox.
- Enjoy all the money you saved on an amp!
[
robg adds: Not being in any way musical, I have no way to test this one!]