VLC media player is a highly portable multimedia player for various audio and video formats (MPEG-1, MPEG-2, MPEG-4, DivX, mp3, ogg, ...) as well as DVDs, VCDs, and various streaming protocols. It can also be used as a server to stream in unicast or multicast in IPv4 or IPv6 on a high-bandwidth network.
Input media: UDP/RTP Unicast, UDP/RTP Multicast, HTTP / FTP, MMS, TCP/RTP Unicast, DCCP/RTP Unicast, File, DVD Video, Video CD / VCD, SVCD, Audio CD (no DTS-CD), DVB (Satellite, Digital TV, Cable TV), MPEG encoder, Video acquisition.
This is my favorite program, EVER.
And I know a lot about encoding and decoding video (and audio, and containers, and subititles, etc.).
Only one complaint: It has problems with some subtitles (ASSA and SSA, I believe).
For a list of things that it can do, check out the Wikipedia article