One of the first games released for the Dreamcast, House of the Dead 2 is an instant classic.  The game is identical to the arcade smash with the inclusion of exclusive Dreamcast levels, and the graphics and detailed zombies will leave your mouth watering.  The voice acting is pretty bad though, making the voice acting from the first Resident Evil look like Shakespearian acting, but that is overlooked by the shoot 'em up action.  A light gun is strongly recommended, but the standard DC controler works just as well, somewhat.