Audacity, the free and open-source audio editor (also available for Windows and Linux) is in need of some Mac-love. Whilst Mac OS X users can get themselves either the stable v1.2.6 release or a 1.3.3 beta version, development going forward is a little uncertain at the moment. According to the Audacity forum, there are no active developers on the project with Macs, and the latest beta (v1.3.4) is not going to appear on the Mac platform until "someone [with a Mac] volunteers to compile it, and sort the remaining bugs out." Yikes!
If you're interested in working on the project, then you'll want to head over to the Audacity 'Get Involved' pages for more information on how to get your hands on the code. Even if you're not a developer, if you're able to contribute via another means (such as with documentation, testing or donations), I'm sure the Audacity team would welcome you with open arms.