You can donate your mattress to The Salvation Army, which picks up donations in many locations. Other options include contacting a local furniture bank or listing the mattress on Freecycle.org.
Call The Salvation Army at 1-800-728-7825 for more information about mattress donations. The Furniture Bank Association of North America offers a list of locations at FurnitureBanks.org. Pickups for donations are usually available within a 15 to 20-mile radius of each furniture bank location. Freecycle.org is a nonprofit website for people to donate and receive items for no cost. It helps people to recycle items and keeps useful products from going into landfills.