MPlayer

MPlayer is a powerful commandline media player.

Installation
Install :

GUI tools
There are a number of different GUI's for MPlayer with different levels of functionality. Some notable ones are:

Disable screensaver while playing a video
Using qdbus (Qt/KDE):

For systems using xscreensaver alone:

My video won't play!
MPlayer uses USE flags to enable and disable support for various codecs. If a particular type of file will not play in MPlayer, first check which USE flags MPlayer was emerged with. Update the USE flags to support for the format you are trying to play and recompile.