ffmpegx is shareware which is annoying since its free software underneath. Alternatively, you can build the command line version, which is better in my opinion. I'm following these directions:
I tried using MacPorts but that didn't work. :-(
Once you get your MPGs and AVIs converted to FLV, you can use FlowPlayer to watch them!Wow. I just realized that Fabrice Bellard is the author of ffmpeg. He's also the author of QEMU. How cool is that?