OGG Theora

Theora is a video codec being developed by the Xiph.org Foundation as part of their Ogg project. Based upon On2 Technologies' VP3 codec, and christened by On2 as the successor in VP3's lineage, Theora is targeted at competing with MPEG-4 video (e.g., H.264, XviD and DivX), RealVideo, Windows Media Video, and similar lower-bitrate video compression schemes.

While VP3 is patented technology, On2 has irrevocably given royalty-free license of the VP3 patents to everyone, letting anyone use Theora and other VP3-derived codecs for any purpose.

The combination of the Ogg container format, Theora video and Vorbis audio allows for a completely open, royalty-free multimedia format. Thus, it is of great interest to the free software and Free Culture movements. Previous multimedia formats (e.g. MPEG-4 video and MP3 audio) were patented, meaning that Free Software players implementing them were legally vulnerable, because paying license fees for software freely available for everyone to use anywhere for anything is often hard, if not impossible to do, regardless of whether licensing fees are rarely collected or not.

-->infos http://en.wikipedia.org   

Main players for OGG & Vorbis formats:

VLC Media Player                 | |

helix player, Mplayer, Xine   

The VLC media player is an open source media player, distributed under the GNU General Public License. It is a highly portable multimedia player that supports many audio and video codecs and file formats as well as DVDs, VCDs and various streaming protocols. It is able to stream over networks and to transcode multimedia files and save them into various different formats.
It is one of the most platform-independent players available, with versions for Linux, Microsoft Windows, Mac OS X, BeOS, BSD, Windows CE, and Solaris, and is widely used with 23 million downloads for version 0.8.5[1]

-->infos wikipedia: VLC Media Player   

download VLC Media Player