Mplayer is the best media player there is. It just is. One just downloads it and it just works, no codecs installation, no registry shit, nothing. It can navigate with the arrow keys (up/down, left/right, pgup/pgdn), it can open unfinished downloads, and can repair/redo their index and navigate while the file is still downloading - awesome. It's very lightweight.
It can play almost anything, no matter how broken it is. It comes with a console tool called mencoder that can encode stuff. It's very hard to use, but it works. It has a GUI for windows and mac os x.
The problem is that the development is a bit forsaken. Sometimes the GUI crashes, there a few bugs, the integration with the OS is not that perfect (I had issues with Vista effects and Mac OS X graphics cards switching)
I just found that the MplayerX (+GUI) is available in the App Store, perfect. The GUI is very slick.
Verdict: highly recommended.