Rock player.
It's the only media player (I know of) with its own codecs included so it can play loads of different videos. It uses the open source ffdshow.
I read on the XDA forums it started off as as a VLC conversion. Anyway it's just like VLC on the PC in that you don't need to install codecs to play the videos, it just handles all that itself.
I was so impressed I went to purchase it, but I thought $10 was a bit steep, if they reduced the price I'd buy it.