Here's a nice multiplayer/singleplayer first person shooter. The graphics look old but that's because the game is based on an old Quake engine. Graphics can be improved by downloading a Hi-Res texture pack (which I've linked to below)
DDay Normandy:
DDay Normandy is a fun, addictive online multiplayer and offline singleplayer botmatch 3dshooter game. Axis vs Allied Currently there are 7 different teams you can play as, each with different weapons, skins and maps: USA, Germany, Great Britain, Russia, Poland, Japan, and Italy, and there are lots of maps for each team.
Video:
You can see a video of gameplay HERE
Download:
You can download the game
here or via Desura/Moddb HERE
You can get a hi res texture pack with loads of maps included HERE and other mods for the game HERE