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:

http://stephenjungels.com/jungels.net/articles/ffmpeg-howto.html

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?