Have you ever needed to record a podcast, edit or mix some sound effects, or edit some music for a video slideshow? Many pros use Adobe Soundtrack, but Audacity is a great alternative. It seem to have more features than Garageband, and it runs on Mac, Windows, and Linux. You can download it for free @ http://audacity.sourceforge.net/download/. You can also use it for amplitude enveloping and fade effects.
Audacity: A free alternative to Adobe Soundtrack