BZFlag is a totallyawesome 3d first-person tank shooter game with tons of flags that give you power-ups (laser, shockwave, stealth, etc.) that also happens to be massively multiplayer and cross-platform, as well as free! :D
Person 1: I think I shall play BZFlag...
Person 2: Go for it.
*two minutes later*
Person 1: WA HA HA I'M HAVING SO MUCH FUN!!!