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:
Via Thingamababy.
Don't radar guns cause cancer? Hmmm... Great kids' toy.
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.kipkay.com/
http://www.edparadis.com/radar/