OSX media player? Try MPlayer. So far it has played everything I've thrown at it, except for realmedia files, which I can do fine in realplayer. Full screen option. Works nicely in the background, with a play screen without any ugly bars on the side, so you can watch it in the background ok. You can controll it from a command line interface too I guess, if your into that...
Just need that, and a couple of codecs.