FREE Audacity Team, audacityteam.org
One of the longest-standing free apps for OS X, Audacity is the perfect tool for making quick edits to audio files. The most recent version, 2.1, adds real-time previews of Audio Unit and BST effects. It also features a new noise removal tool, and the Spectral Selection feature allows you to click and drag to select frequency ranges to manipulate, as well as time periods. If you need to edit audio on a Mac, this is the best place to start.