Valve's multiplayer shooter Team Fortress 2 is brilliant.
If you haven't experienced the wildness of this latest installment, I strongly recommend that you do...it's awesome. In promoting their "Gold Rush" expansion for the videogame, Valve is allowing free play from May 2 - 4. Sign up for an account on Steam and a full version will be accessible. Of course, you can download Team Fortress 2 on Steam or even purchase it as part of The Orange Box.
The Gold Rush expansion includes one new map, and new weapons for the Medic Class.