FFmpeg

FFmpeg is used mainly to convert and stream video. FFmpeg uses libavcodec. FFmpeg is developed under Linux, but can be compiled and run under most operating systems.

How to get it
svn co svn://svn.mplayerhq.hu/ffmpeg/trunk ffmpeg cd ffmpeg ./configure && make && make install

Formats
FFmpeg supports many formats and containers, including: mpeg-1, mpeg-2, DIVx, H.263, H.264, MP2, MP3, and MP4.

How to use it
Here's an example how to convert example.dv to a .mpg file: ffmpeg -i example.dv -f avi -vcodec mpeg4 -b 1800 -g 300 example.mpg

FFplay
FFmpeg also includes FFplay, a media player and streaming client.

FFserver
FFmpeg also includes FFserver, used to stream media to clients over a network.