Air bags... a lot of folks here on OSO run them. Two major brands are Air Lift and Firestone.
I have Air Lift 1000s on my second truck, separate air fill fitting for each bag, no on board compressor. (Separate fittings are important. If you have one air fill fitting split to both bags, and if you have a load in the bed of your truck, that shifts in transit, the air bags can disperse air from one side of the truck to the other, and put you in an off camber situation. If you are using them for towing either a tag along, or gooseneck/fifth-wheel, your weight should be evenly set on the center of the long axis of your truck, you can get away with a single air fitting for both, however, its just as easy to have too, and safer in my opinion.) I have the air fill fittings (just like a tire valve stem) 'hidden' as now being my rear upper license plate bolts (pretty trick) and others hide the fittings inside the gas cap drilled into the plastic housing.
When you are not towing, you basically now have an adjustable anti-sway bar for the back of your truck. It really helped make both my trucks (previous 1500 Tahoe and currenty 2500 Avalanche) handle better. Firestones have to be kept at 5 lbs.min.at all times; Air Lifts are 10 lbs.min. at all times... to keep from creasing. Even when I put the boat away for the winter, I leave them at 30 lbs. per side just for the extra support and anti-sway, especially in a winter fish-tail situation.
Towing, it gets rid of any squat the truck has when the trailer is attached. I am also running a weight distributing hitch, which makes towing really nice. Everyone buys their truck on towing capacity but never really factors in their cargo when towing either. By the time you load up your 3 coolers, 2 suitcases, 2 toolboxes, and 4 people in the cab, your truck has a lot of weight in the truck itself. The airbags really help distribute that weight from the trucks factory suspension, to a very capable weight transfer (airbags) to the frame.
Some more info:
http://www.offshoreonly.com/forums/s...hlight=airbags
http://www.offshoreonly.com/forums/s...hlight=airbags
Also, OSO member Troutly is a wealth of information knowing both the law about towing and just about every spec for every truck in his head.