www.bumpfirestocks.com
Technically they are called Slidefire Stocks, and a friend tested one of them and a full auto AR, and the Slidefire equipped AR was actually a tiny bit faster than the full auto.
That being said, they are a fad. A local gun shop bought 3 of them, sold 2 real fast, and the third one hung around for more than a year before it sold. I can burn up ammo fast enough with a semi-auto, so I never got one. None of the 5 ARs I own have one.