Although MPlayer is one of the most popular and best looking players for Mac and Linux, it's not exactly the most feature rich media player out there.
MPlayer OSX Extended is one developers attempt to redesign the GUI and add new and more powerful features that out MPlayer on a level with other players of its ilk. These include a host of new features such as many ore video filter options (such as Postprocessing, Deinterlacing etc), support for styled ASS subtitles and audio and video equalizers.
It also now includes a function for taking screenshots in MPlayer which means that you can take shots of individual frames and scenes. However, it seems a result of bloating it with these features has slowed down MPlayer a little meaning it takes more time to open movies and files. However, this isn't enough to take away from the fact that this is a considerably improved MPlayer.
Easy to use, packed with features and still the great looking app it's always been, MPlayer OSX Extended is a definite improvement on the original.