Aside from commercial alternatives, the preferred open source audio editor tool is Audacity. Early releases of Audacity were unquestionably rough around the edges, in terms of both stability and features, but thanks to the efforts of the open source community, the software is steadily improving.

Version 1.2.5, released this week, is the new stable version, with Intel Mac support, bug fixes, and now FLAC audio support. (Seems the FLAC support alone is worth keeping it on your hard drive, even if you prefer other tools.)

The 1.3.2 beta is where things start to get more interesting:

1. New selection bar and improved selection tools
2. Dockable toolbars
3. New “Repair” effect, other improved effects
4. Auto-save and automatic crash recovery

Audacity Homepage
New 1.3 Features