Audacity is a popular freeware multi-track audio editor and recorder.
X Lossless Decoder (XLD) is a tool for Mac OS X that is able to decode / convert / play various ‘lossless’ audio files. The supported audio files can be split into some tracks with cue sheet when decoding. It works on Mac OS X 10.4 and later. XLD is Universal Binary, so it runs natively on both Intel Macs and PPC Macs.
MP3 Diags is a GUI-based application that allows end-users to identify issues with their MP3 files, fix some of the issues and make other changes, like adding track information. It also lets you “look inside” an MP3 file.