The x264 QuickTime Codec is a QuickTime Image Compression Component which uses the free, open source x264 H.264/AVC encoder. It can produce H.264 encoded video faster than the Apple-provided H.264, with more configurable options and better quality.
iSquint is an iPod/TV video conversion app for Mac OS X.
MediaFork is a GPL’d, multiplatform, multithreaded DVD to MPEG-4 ripper/converter, available for Mac OS X, Linux, and Windows. It is based on the source code to HandBrake
MediaPipe is a flexible framework to manipulate media on MacOS X. It allows you to build decoding, filtering, encoding and even streaming pipelines that correspond exactly to your needs.