Can someone recommend some sound deadening material?

Lowes sells a roofing product called "Peel and Seal". It's a self adhesive Tar-like material with foil on the outside for heat resistance. I'm told it's the same material that Dyna-mat started out with before they developed thier own. I did the rear speaker deck of my Duster. With the car off the front speakers are louder than the rear, but as soon as I start the car and drive the fronts get lost but the rears sound awesome. I haven't gotten around to doing the doors yet, but I plan to them when I do the carpet and replace the original padding with this stuff. A friend of mine is really happy with Lizard Skin paint on coating on his Cobra kit car, but it's kind of pricey.