Hello, I use Rockplayer. It plays many kind of files, including avi, flv and mkv. However it is not free. It costs $ 9.99. But I think it is worth it. You can also try a free version, that has some ads in it. The only "glitch" that I have found is that it lags a bit when scrolling trough a movie. But it still is great.
Hope this helps.