MacMP3Gain is an AppleScript Studio application which wraps up the command line version of aacgain (derived from mp3gain) in a quaint GUI. It is free and open source. You may download either the application itself or the entire project including the source code.
What is new in this release:
Fixed mixed up Radio Gain Mode and Undo buttons. Fixed progress tracking. Built on Snow Leopard - should run on Lion (untested).