x264 is an open source, free, h.264 / AVC video encoder. It is one of the most advanced publicly available tools for encoding h.264 video. It is also a High Profile AVC encoder, one of very few publicly available. The code is written from scratch by Loren Merritt, Laurent Aimar, Eric Petit, Min Chen, Justin Clay, Måns Rullgård, Radek Czyz, Alex Izvorski, Alex Wright, and Christian Heine and is released under the GNU General Public License.
For an average user to use x264, the help of a GUI tool to interface it with is required. Tools such as StaxRip and MeGUI fill this need.