http://www.videolan.org/vlc/

VLC - the cross-platform media player and streaming server VLC (initially VideoLAN Client) 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.