Although it is a command-line based player (mainly) and the official GUI is so poorly designed :P in comparison with the features that you can use while on the command line, the official GUI (known as "gmplayer") only includes about 1-2% of the features that it comes with!. It is almost disrespect to this amazing app!.
Like Xine, it is a separate project and is not based on Gstreamer or any other framework, although they use the ffmpeg codecs nonetheless. From adjusting subtitle delays, audio/video delay, few dozens of video and audio enhancing effects, web browser integration (via plugins), playing DVD/ Blue-ray or any other codecs, etc... you name it.. and MPlayer have it!.
Anyway, enough boasting, if you use Ubuntu 10.10 then you can easily install it by entering the below command in your Terminal (make sure you have enabled universe, multiverse repositories).
sudo apt-get install mplayer
This should install the official GUI called the "gmplayer" as well and if you want the mozilla addon then issue the below command also.
sudo apt-get install mozilla-mplayer
There are hundreds other projects that are based on the MPlayer (such as the SMplyer which is a Qt based pretty amazing GUI!), check them out in this page.