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.
Perian is a free plugin that enables QuickTime to play almost every popular video format.
QTCoffee is a set of command-line Mac OS X utilities for manipulating QuickTime movies and other QuickTime readable media.