HandBrake is an open-source, GPL-licensed, multiplatform, multithreaded video transcoder, available for MacOS X, Linux and Windows.
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. The VLC media player will take care of things there. It is used in conjunction with Handbrake.
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)