x264 - a free h264/avc encoder
x264 is a free software library and application for encoding video streams into the H.264/MPEG-4 AVC format, and is is released under the terms of the GNU GPL.

The code is written from scratch by Laurent Aimar, Loren Merritt, Eric Petit (OS X), Min Chen (vfw/asm), Justin Clay (vfw), Mans Rullgard, Radek Czyz, Christian Heine (asm), Alex Izvorski, Alex Wright and Jason Garrett-Glaser.

http://www.videolan.org/developers/x264.html