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's new in version 2.1
Fixed mixed up Radio Gain Mode and Undo buttons. Fixed progress tracking. Built on Snow Leopard - should run on Lion (untested).
ReleaseDecember 22, 2011
Date AddedDecember 22, 2011
Operating SystemsMac OS X 10.4 PPC, Mac OS X 10.5 PPC, Mac OS X 10.4 Intel, Mac OS X 10.5, Mac OS X 10.6 Intel, Macintosh, Mac OS X 10.4, Mac OS X 10.7, Mac OS X 10.5 Intel