Windows

There are far too many media players available for windows, most of which are limited to a few file types that they can play well and your just out of luck for any unsupported video format that you may come across.  I have faced this problem many times and finally there seems to be a pretty reliable solution!  

all you need is: 

The codec pack will cover just about every video decoder you could possibly need, and has many settings if your into that sort of thing.  VLC is a great media player to have for transcoding, playing DVD's, and repairing broken AVI files.  Forget your other players and set Media Player Classic or VLC as your default video player and you'll save yourself some frustration.

Mac OS X

 

Ubuntu