DIVE INTO SOFTWARE / VIDEO /
xine
G¸nter Bartsch & other developers
http://xinehq.de
*nix
GPL
From the website:
"xine is a free multimedia player. It plays back CDs, DVDs, and VCDs. It also decodes multimedia files like AVI, MOV, WMV, and MP3 from local disk drives, and displays multimedia streamed over the Internet. It interprets many of the most common multimedia formats available - and some of the most uncommon formats, too."

Xine is for video what winamp and xmms are for audio: a simple and easy-to-use point-and-click application for playing video files. Its' features boast of numerous input and output plugins including a nice post-effect plugin called GOOM which generates images from music. Additionally, xine is able to play numerous streaming protocol formats, everything from HTTP and MMS, to PNM and RTSP.