If you need to convert those home movies of birthday and xmas parties from years gone by, you need a transcoder of one kind or another. Linux has a few.
Handbrake. Multiplatform video / audio transcoder that handles almost any format you like. Its fast, stable and actively developed.
It supports any DVD-like source: VIDEO_TS folder, DVD image or real DVD, and some .VOB and .TS files, Any multimedia file that libavformat reads and libavcodec decodes.
Handbrake outputs the following formats: MP4, MKV, Video: MPEG-4, H.264, or Theora, Audio: AAC, MP3, Vorbis or AC-3 pass-through.
Transcode. Transcode is a command line utility, for transcoding video, audio, and container formats. It includes a variety of video and audio filters, including video de-interlacing, audio resampling, framerate conversion, different video resizing algorithms, smoothing, sharpening, noise removal, and cutting.
Transcode decodes / encodes audio and video formats: MPEG-1/2, MPEG-4 (-part 3) DivX / XviD, Quicktime, MPEG-PS (decode only), MPEG-1-layer-1/2/3 audio, AC3 audio.
A number of video and audio processing filters are available for: video de-interlacing, audio resampling, framerate conversion, smoothing, cutting. Transcode can import DVDs on-the-fly, or record from Video4Linux devices.
Transcoder. To really muddy the water and confuse things, we also have Transcoder. It doesn't use Transcode but FFmpeg at the backend and GTK+ for a GUI front end.
FFmpeg. A fast video / audio converter that can also grab from a live video / audio source. Using a command line interface designed to be intuitive. FFmpeg tries to guess the required parameters automatically. Usually only specifying the target bitrate.
FFmpeg converts any sample rates to those desired. It can resize video on the fly.
WinFF. A GUI front-end for FFmpeg. Allows conversion of video and audio formats. Once installed its easy to use.
Mencoder. The other half of mPlayer. The awesome all round media player for Linux. Mencoder will convert most formats. Mencoder is one of the big boys of Linux transcoders.
Linux Video Converter Rudd-O. Linux Video Converter. The simple video transcoder for Linux. Choose your video, select the new format, hit OK and presto, it's done. Linux video converter makes it easy to convert your videos to several popular formats to archive or upload into video sharing sites, such as YouTube or Vimeo.
It uses the powerful Mencoder transcoding suite.
VLC. VideoLanClient is a media player and a video network server. VideoLan was originally developed to stream media over a LAN (local area network), hence the name VideoLan. The VideoLan Client can also convert media to another encoding. VLC has many codecs inbuilt by default.
You have a few to choose from. Go have a look!