HandBrake is also a freeware software package which does video format conversion. First released in 2003 (I think?) this handy bit of kit can pretty much turn anything in the video world into anything else. This is incredibly useful if you collect movies or other video media, given how many formats there are. MKV? Sure. Ogg Vorbis? Why not. Apple MPEG, as distinct from other MPEG? Indeed.
Ths software is available on Mac OS X, Windows and Linux. It uses a whole list of open source libraries to do its magic. You can queue up dozens of conversions and leave them running using its built in queue manager; you can set up standard conversion operations for a queue so you don't have to manually configure each job, and so forth.
In addition to handling basic video codec and container conversions, HandBrake can convert attached audio tracks between all manner of formats, from straight 2-channel stereo to WAV to MP3 to Dolby Pro Logic [PCM} and on up to AC3. It can also handle upmixing and downmixing to get all that right. Finally, it can be used to handle subtitles as well - whether by embedding them into the container as text tracks, or if necessary burning them into the video permanently in your selected font and style of choice.
HandBrake is available at handbrake.fr. Its maintainers are incredibly conservative in its versioning - on the Mac, it's been out over a decade and the current version is 0.10.2.