VideoLAN's VLC is the premier video and audio player. It is the most widely used video player on Windows and has made many a Mac user's life easier since it arrived on the Mac.
The first thing that you will notice is that VLC is made up of two windows when you are playing video: the video player and the control window. The video player has a progress bar at the bottom that shows the time left in the movie and when you move your mouse over the window when in full-screen mode, you get some quick controls that fade away when you move your mouse away from them. The control window has the same controls, and you can pop-up an equalizer and playlist information. From there, you can choose to add more files to the playback queue or simply to automatically repeat files after completion. One thing that I did not like is that VLC seems not to remember certain settings when you quit it. My video auto-repeats when it finishes, and I have tried to disable that to no avail. Still, it is not a big problem.
Performance-wise VLC does a pretty good job playing most standard-definition files. It plays HD files well, but at times, there is some choppiness. The great problem that VLC for Mac faces is that it still does not support hardware acceleration. The Windows counterpart does, and thus it performs much better. Another problem I had with VLC is that subtitle font size is a bit weird to set up. I wish it had a more intuitive way to set the size. Other than that, the player is great. It supports virtually every format that you can think of and plays most with excellence. It has a web interface for you to control it from a web browser or iPhone. It can convert files to other formats and it can even be configured to stream video to other devices.
In short, VLC is the greatest video player that is lightweight and powerful. It has some nuisances, but what doesn't? Life wouldn't be the same without it right now. It has an extensive hotkey support and some power features.