What's a good simple shooter game for PC?

My dad is a big fan of simple shooter games, lots of time they include zombies or simple flying games where there aren't a lot of controls. I'm a fan of World of Warcraft but something like that is much too complex for his taste. I was wondering if anyone had any suggestions for games that could be bought in store or downloaded online that are entertaining.

Also, if anyone has played Diablo, if they could let me know how the gameplay is on that as well. He showed interest in that game but again, i'm not sure of the complexity of the controls.