There is no excuse for not having these games downloaded on your computer!
Alien Arena is a first person shooter game with a Sci-Fi twist, and unbelievable graphics. It incorporates the best elements of games like Quake III and the unforgettable Unreal Tournament, yet the never-seen-before ideas will blow you away.
If you prefer strategy games you have to download D-Day Normandy. It is a 3D war game which depicts the D-Day landings in Normandy in 1944. This game has garnered critical acclaim for the the strategical elements of this game.
Frets on Fire is an open source game similar to Guitar Hero. You create and play songs with a keyboard according to the markers on your screen. Rock on!