Nine, there are a couple that are very similar. The one I use is the Clickety Click, the other is the Cricket. I think BW sells one of them, and Three Rivers selss them both.
Basically they have a small piece of spring steel, attatched to your bow limb. From it runs a cord to your bowstring. You set it so when you are at full draw and apply back tension, it "clicks", then you immediately release.
I think it adds much more to my shooting than just a draw length check. It makes my focus and concentration much greater before the shot.