The most common "weapon" used in a sumobot competition is an angled blade at the front of the robot, usually tilted at about a 45-degree angle towards the back of the robot. This blade has an adjustable height for different tactics.
THERES NOT MUCH TO SEE BUT I LIKE THE IDEA OF THE HAMMER, IN THIS VIDEO. AND THEY ACTUALLY USE ONLY PIECES THAT COME FROM THE NXT TOOLKIT, AND NOT FROM OUTSIDE.