FFmpeg is an extremely powerful and versatile command line tool for converting audio and video files. It is free and available for Windows, Mac and Linux machines. Whether you want to join two video files, extract the audio component from a video file, convert your video into an animated GIF, FFmpeg can do it all and even more.

Full article here:
Useful FFmpeg Commands (Digital Inspiration)
Related: OS X installers (second line below contains link to newer version at this writing):
Run ffmpeg On Mac Without Compiling Anything (peppoj.net) (UPDATE to this article)
FFmpeg installer for OS X (Rudix)