VideoLAN VLC is a great cross platform media player ideal for networks. CODECs for most formats are included with the player so it can be easily deployed without needing extra installers.
As it is self contained it is very easy to repackage as an MSI for deployment or it can run straight off the network without installation (although it is a bit slow to load this way).
VLC runs on Windows, Mac and Linux so it can provide a uniform interface across platforms.
The latest version, 0.8.6d fixes some security problems and improves playback on Mac.