Mac OS X
program published by Rogue Amoeba
software. It does exactly what you'd expect: Audio Hijack intercepts the sound output from any specified program and dumps it to an AIFF file.
The program sports a ton of ways to edit its output including a host of Digital Signal Processors: a software preamplifier, an echo filter, a ten band graphic equalizer, a balance filter and a gain doubler. The program also sports timers to grab internet radio broadcasts and the like.
The program is powerful because it's uses are so many and varied. Want to rip that game music to remix it? How about taking the audio from a band's live DVD? Want to record a realplayer stream? Any of this is possible and very easy.
The thing that makes Audio Hijack so convenient to use is that it doesn't lock up the rest of your computer to do its thing: it only pulls the audio from the one specified application, so you can launch iTunes, start to record something and still play movies in the foreground without any fear of the dialogue getting sucked into your recording.
Audio Hijack is shareware and is priced (reasonably, in my opinion) at $16US. The program is minorly crippled before registration: recordings automatically end after thirty minutes.
A more advanced program, Audio Hijack Pro, offers the ability to record directly to MP3, to have up to 100 signal processors active at once, to hijack applications as they're running (instead of launching them through Audio Hijack itself), the ability to change a recording's file name (they're named as date and time of the rip with the standard version) as well as other advancements. Audio Hijack Pro costs $30US.
Both programs are available at http://www.rogueamoeba.com