NXT Claw Striker
After I insulted my NXT for being physically inferior to my cat in just about every way, it sulked back down to my den and contemplated its next move. Hey, what if it stole a play from the cat's book, and tried striking quickly with a claw?
This Claw Striker robot uses the ultrasonic sensor to turn around and locate a target, move to the proper range (forwards or backwards as necessary), then strike quickly with it's claw-tipped whip-like arm.
Here's a video. No, it's not brave enough to take on the cat yet, even if I would let it (please don't try with yours either).