With MPlayer you won't have to cross your fingers and hope you have the right codec every time you open a media file on your computer. MPlayer is a simple yet great media player that supports more than 192 video and 85 audio codecs natively, which means it'll probably be able to open just about anything you throw at it.
Like I said, the interface in MPlayer is not exactly brilliant. It's quite basic, almost minimalist. But the program includes a plethora of great features to make up for it: fast response when playing videos, very low on system resources, support for subtitles, ability to take screenshots and much more, plus an extensive support for keyboard shortcuts.
Easy enough for newbies, powerful enough for experts, MPlayer is an excellent media player with support for literally hundreds of audio and video formats.