ffmpegX - Encoder for Mac OSX
Without a doubt ffmpegX (currently version 0.0.9v) is an essential OS X application. If you need to encode or transcode video on a Mac then this is the gadget for you. It's a front end for a bunch of UNIX video encoding tools. It's every easy to use once you've got your head around the install procedure. The encoding is very quick, even my humble Powerbook Pismo G3 500Mhz can encode in an acceptable amount of time. If you're lucky enough to have a G5 is simply flies along!
Talking of slow computers, I find ffmpegX a lifesaver when I'm trying to play a video which is too intense for my poor little G3 cpu. I simply transcode to a more acceptable setting and without losing too much quality I have smooth playback on my old laptop.
ffmpegX is an opensource app so feel free check it out!