While there are plenty out there, it's hard to find one which gives access to all mplayer options.
This is what Xt7-Player aims to be, a complete graphical interface to mplayer, focusing on usability.
I strongly encourage users to use this board to submit bugs, to request new features, post impressions and constructive critics.
Here are some screens from the new gambas3/qt4 version
Old and unsupported Gambas2/qt3 version:
Video controls and video area in a single window:
Support for local,network, and 'exotic' protocols:
Support for multiple profiles
Misc video options
Misc audio options
Tweaks
Misc options
Network options
Local & Global Hotkey managment
Freedom to bind actions to mouse and/or keyboard buttons.
Subtitles and Osd message configuration
Audio Video codec priority support
Support for a wide range of audio and video drivers with their own sub-options
"Stackable" Audio and video filters managed via drag'n drop
Each audio and video filter has it's own options to configure
Mplayer has toons of filters, if you need something which hasn't been implemented in Xt7, you can write your own:
Video Equalizer
Support for multiple audio And subtitle streams you can switch with a click
Fullscreen interface with popup and dropdown controls:
Per window Settings managment as well as application wide
Media library and playlist managment
Iso/VideoTS DVD navigation support
Cover manager and downloader via Amazon web services
Media tags reading and editor via audiotag
Inline search configuration
Shoutcast Radio by genre and by search:
(insert image here)
Audio Cd:
(insert image here)
Xt7-Player is written in and requires Gambas.
Have fun!
this is amazing. mplayer based frontend with library support and tons of feature. this could be the media player of linux (like windows media player of... well .. windows 
)