MPlayerX is an open-source media player based on the MPlayer project. I have to admit that I had never been a fan of either the Windows app or its Mac OS X port. I don't remember exactly why I didn't like the Windows version, it probably had to do with how good VLC was for me at that time, I guess. The Mac OS X version, MPlayerX, though not often updated, has shown me that it has promise.
Let me describe what exactly this app does and what is good about it. MPlayerX has great support for video files. It opens pretty much every file that you can have. I just tested it with MKV, MOV, AVI, MP4 and FLV files, and it opened them all. It has quite a few interesting features. One of them is the fact that the app has great subtitle support and that you can customize everything about it. MPlayerX picks the best encoding for subtitles, so no conversion is needed. It supports the Apple remote and multi-touch gestures. Another interesting feature is that the player will remember where you stop watching your files and will resume from there next time.
In terms of performance, MPlayerX performs quite well. I didn't notice any skipping or quality problems with my files. In fact, it worked just as well as Movist, my favorite player these days.