VLC is a free cross-platform media player.
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.
For more information about what features are supported on your operating system, please see the full features list. You can also have a look at some VLC screenshots.
To get a glimpse of its popularity check its download statistics. I have been using V-LAN for the past few months and its one player which has never let me down. Plays almost any known music audio or video file format and also looks nice with its different skins. Best of all anyone can make their own skin. It can be used to stream video on to the network. A useful feature is that it can convert video formats from one to the other. I have been able to convert a avi file to a mpeg format using it. You might just have to see its feature list to believe me. The possibilities of this player seem endless. Just WOW!