x264 is a free library for encoding H264/AVC video streams. The code is written by Laurent Aimar, Eric Petit(OS X), Min Chen (vfw/nasm), Justin Clay(vfw), Måns Rullgård and Loren Merritt from scratch.
x264 is one of the most advanced and best quality AVC encoder, and it was the winner of Doom9’s popular head to head codec comparison in 2005.
x264 is supported by a wide variety of open source projects, including ffmpeg, ffdshow, Handbrake, VLC, MPlayer, and VirtualDub.