gmplayer the built in one isn't too bad. There may be a boring looking skin for it. I've heard of KMplayer and Kplayer which are KDE frontends... and mplay seems to be a curses based version.
gmplayer's OK mostly, but I'm tired of fiddling with the volume dial. It's a slightly broken idea when mouse control is needed. A slider would be more useful.
It uses the same codecs [Especially ffmpeg] and will play just about anything mplayer does1. And it has a passable gui.
And it's cross platform. OSX/Win32/Linux/Other
1I'm assuming you're not wanting a really obscure codec/
I've heard of KMplayer and Kplayer which are KDE frontends...
and mplay seems to be a curses based version.