kmplayer is a front-end for mplayer, together with
desktop files to embed it in konqueror.

The front-end is not very useful, but the embeddable
part means lots of windows video sites suddenly start
working in konqueror.

Care must be taken for the audio output. You can either:
- install an sdl with arts support and use an mplayer with sdl audio output,
- use an mplayer with direct arts support,
- disable arts audio output in kde entirely 
(control panel, multimedia, sound io, no output)

The last option is possibly preferable, as kde arts does not yield real time
performance.