FFmpeg is a complete, cross-platform solution to record, convert and stream audio and video. It includes libavcodec - the leading audio/video codec library.
HandBrake is an open-source, GPL-licensed, multi-platform, multi-threaded video transcoder, available for MacOS X, Linux and Windows.
MPlayer is a movie player which runs on many systems. It plays most MPEG/VOB, AVI, Ogg/OGM, VIVO, ASF/WMA/WMV, QT/MOV/MP4, RealMedia, Matroska, NUT, NuppelVideo, FLI, YUV4MPEG, FILM, RoQ, PVA files, supported by many native, XAnim, and Win32 DLL codecs. You can watch VideoCD, SVCD, DVD, 3ivx, DivX 3/4/5, WMV and even H.264 movies.
VLC is a free and open source cross-platform multimedia player and framework, that plays most multimedias files as well as DVD, Audio CD, VCD, and various streaming protocols.