Wikipedia
Perian
Perian was an open source QuickTime component that enabled Apple Inc.’s QuickTime to play several popular video formats not supported natively by QuickTime on Mac OS X. It was a joint development of several earlier open source components based on the multiplatform FFmpeg project's libavcodec and libavformat, as well as liba52 and libmatroska.
It has been featured as the "Download of the Day" on Lifehacker, as well as on several popular blogs including Ars Technica and The Unofficial Apple Weblog.