BladeEncX is a Mac port of Tord Jansson's venerable BladeEnc MPEG-1 Layer 3 "MP3" encoder to Mac OS X. Unlike previous ports which were strictly Terminal-only, this one includes an Aqua-based interface to its options (BladeEncX), along with a "droplet" for doing repeated batch conversions once you've chosen and saved options (BladeEncRunner). That sure beats firing up iTunes just to tweak a song. BladeEnc offers multiple bitrates and compatibility with AIFF and WAV (PCM and ADPCM) files.
Mac OS X 10.2.8 or later