http://www.videolan.org/
VLC Media Player is a free open source cross-platform media player.
VLC Media Player has support for a large number of multimedia formats without the need for additional codec.
It can also be used as a streaming server with extended features, i.e. video on demand, on the fly transcoding…)
Video streaming is outlined here:
http://www.videolan.org/vlc/streaming.html
I haven’t used it yet but saw it recommended on the dl.tv video podcast episode 93 and it sounds pretty cool!
http://dl.tv/blogs/digitallifetv/archive/2006/09/07/20538.aspx