Love VLC's ability to play pretty much any media file on earth. Their iPad app is also seriously handy.
VLC Media Player
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Just about the most versatile media player out there. It doesn't require any external codecs and still it is able to play just about any media file known to man. Awesome set of features built-in but is easy to use at the same time. Startup is really fast and the player doesn't use much RAM compared to other players. If you are really oldschool then you will also appreciate command line support! :)
VLC is soooo powerful it hurts. As wel as play as a client it can act as a media streaming server. The beauty is that it's the same software that can do both! The server code even has built in HTTP authentication without the need for an external web server. Simply awesome!
VLC is a champion media player. It's free, open source, works on operating systems you've never even heard of and plays most every file format (including, oddly enough, RAR files)
VLC also offers a bunch of awesome yet little-known features such as ASCII terminal mode, multicast, YouTube video download, a web interface, and automatic audio normalization.
I wish Winamp would have stayed as popular.