Audacity: The Free, Cross-Platform Sound Editor

Audacity® is free, open source software for recording and editing sounds. It is available for Mac OS X, Microsoft Windows, GNU/Linux, and other operating systems.

 If you've ever wanted to make your own mixes or even just take a sample of Stewie from Family Guy annoying Lois to use as a ringtone. Audacity is the software to use. Plug-in support, click and drag interface, multi-channel support. The list of features goes on and on!
Best of all, its FREE!!!