Rui Carmo
VideoLAN is an Open Source Video Streaming solution that supports practically every useful desktop OS out there. The client can also handle disk files, and has become one of the most popular desktop video players for Mac OS X.
Plus it's free, which goes a long way towards minimizing some of its UI quirks.
It can play MPEG, DVD, AVI and pretty much any sub-format you throw at it (yes, including DivX, XviD, etc.) in windowed and full-screen modes, is supported by several Apple Remote utilities, and can be automated in a number of ways.
Resources
Notes:
Streaming video manually from a server (for multicast testing):
vlc -I ncurses -vvv test.vob --sout='#standard{access=udp,dst=239.255.0.1,port=1234}'
Simply open the same multicast address on a client. more examples.