When it comes to audio and - especially - video files, you'd be hard-pressed finding a media player that can beat the versatility and ease of use that VLC offers. This cross platform application can not only play almost any media file available right out of the box without the need for additional codecs, but also functions as a streaming media server and can transcode files from one format to another.
Though not intended to be used as a DVD ripping tool, VLC can certainly get that job done too - if you don't mind the extra steps involved. It also ignores DVD region coding, making it a region free player you can rely on when purchasing movies abroad. Other less conventional features include the ability to do screencasts and record the desktop. Best of all, VLC media player is free open source software with versions available for Windows, Mac OS X, various Linux distributions and more. You can find a complete features list for every platform here, where you'll also get a good sense of the vast number of formats supported.