Although LEGO doesn't like to sell or support weapon-products, let's face it... robotic weapons are lots of fun! Besides the "imagination" factor, weapons can have "practical" uses as well. For example, two or more zamor-shooting, repeating rifles/pistols can make for a great LEGO "Paintball" match! (even if it is a little close-range :) )
You can make many different kinds of NXT weapons. Some ideas include crossbows, pistols, rifles, machine guns, catapults, and slingshots. Below are some resources that you might find helpful if you want to make one.
First, there's Forbidden LEGO by No Starch, which gives instructions for making some neat-looking LEGO weapons. Although it's not NXT-related, it looks like it would be easy to interface the models with the NXT. You can also get some great inspiration from the models.
Some current NXT weapons include my Beam Weapon (scroll down) and robots entered in NXTasy's "Throw Me!" Contest, such as Matthias' Dicke Bertha and Dicke Bertha Typ II.
P.S. See other Robot Inspiration posts here.