serious

Mplayer.

Anyone know of a front-end for Mplayer that's worth installing? Something boring-looking that works properly would be nice.
www.videolan.org/VLC is a nice alternative to mplayer.

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/
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.