The Hot Wheels Radar Gun is a genuinely working radar gun ($25). Here's a commercial for the device, featuring some young kids living an EXTREME lifestyle:
The range of the one we got wasn't too far but not bad. My boys would use it for stuff 6ish feet away. I know it's not good for catching speeders but, we never really tried it on other things. I guess it's time to dig it out of the toy box and change the batteries. The coolest part was that you could change it from real-world speed to hot wheels scale speed. It would tell you how fast that little car was moving if it were a full size car.
http://www.edparadis.com/radar/