Many 20 pound propane tanks are refillable. Bring the cylinder back to the store you bought it from to be refilled.
Both 1 pound and 20 pound propane cylinders can be brought to the Clean Sweep Hazardous Waste Collection or the Appliance / Electronics Collection for disposal. Here is more information on Clean Sweep.
Contact the company listed on the tank and they should be able to reuse or recycling the tank.