How can I record sound playing on my computer?

I want to record sound that is playing on my computer maintaining as much of the original quality as possible. It would be ideal if I could record the sound as MP3 or WMA. I only need the sound, not the screen capturing, as many programs do. Also, free would be nice. And just to clear up potential confusion, I want it to record directly from the computer (i.e. without using a microphone). So, any ideas?