I have always been a huge fan of VLC. It's open source, plays a lot of media files, is relatively light on resources, and runs on just about everything. Especially on Mac OS X, where the QuickTime player is limited and cumbersome, VLC offers an amazing alternative. However, on Windows and Linux it performs its role very well too, and it even runs on BeOS and Syllable (even though those versions are a little outdated). Sadly, I haven't been using VLC as often as I used to now that Windows 7's Media Player received its cleaner interface.
Originally, the project was supposed to have a client and a server, but features from the server side merged into the client side as time went on. The VideoLAN Client name was deprecated, in favour of the VLC Media Player name. Apart from the VLC Media Player, the VideoLAN Project also hosts several other projects, such as libdvdcss and x264.
Verson 1.0.0 of VLC introduces support for HD codecs (AES3, Dolby Digital Plus, TrueHD, Blu-Ray Linear PCM, Real Video 3.0 and 4.0, etc.), more formats (Raw Dirac, M2TS, etc.), instant pausing and frame-by-frame support, and much, much more.