HandBrake is an open-source, GPL-licensed, multiplatform, multithreaded video
transcoder, available for MacOS X, Linux and Windows (...and FreeBSD).

Supported sources:

    * Any DVD-like source: VIDEO_TS folder, DVD image or real DVD
    (unencrypted--protection methods including CSS are not supported internally
    and must be handled externally with third-party software and libraries), and
    some .VOB and .TS files 
    * Most any multimedia file it can get libavformat to
    read and libavcodec to decode.

Outputs:

    * File format: MP4, MKV, AVI or OGM 
    * Video: MPEG-4, H.264, or Theora (1 or 2 passes or constant quantizer/rate
      encoding) 
    * Audio: AAC, MP3, Vorbis or AC-3 pass-through (supports encoding of several
      audio tracks)