bbDEMUX – Free
http://sourceforge.net/projects/macbbdemux
I stumbled across this great piece of freeware when stuck with an editing problem. My client had some MPEG-1 files they wanted to edit together for their website. While I was able to import them into FCPand set the timeline to 320X240, I couldn’t get the audio to import. Turns out with MPEG-1 files, the audio and video are merged into one track (muxed). This great little app does nothing more then split them apart, but that’s all that was needed and its free!