MPlayer is a powerful multimedia player and more - It plays almost anything from RealPlayer to avi. It has a useful and powerful companion for video encoding called mencoder.
There is a MPlayer plug-in for Iceweasel, gecko-mediaplayer. See also: MPlayer on Wikipedia.
Front ends
MPlayer has many independently developed GUIs:
Binary codecs (optional)
MPlayer can use binary codecs to extend support for file formats, see MultimediaCodecs.
Other
Render video as ASCII art: mplayer -vo caca MyFile.mpg or mplayer -vo aa MyFile.mpg
Debian-specific information
http://people.debian.org/~mjr/mplayer.html - The Debian and mplayer FAQ
upstream specific information
other information