When I think of fast-paced multiplayer shooters, I think of first-person games like Halo and Call of Duty. I don't generally think of text-based games.
Little did I know that a text-based multiplayer shooter, specifically Eigen Lenk's Text-Based Multiplayer Shooter, could be so much fun. (Jason tells me that Aliens vs. Predator: MUD was similar, but I haven't played that one.)
It's a breeze to get started, and before long you'll be running through rooms by typing commands, shooting random players, and occasionally getting taken down.