Call your local recycling center and ask if it accepts large aluminum items such as a ladder. Disassemble it if they require it, and put it in your recycling bin if you have curbside pickup. Otherwise, take it to the recycling center and drop it off during business hours. Call 800-Clean-UP for information on local recycling centers.
Donate the ladder to a local nonprofit or charity. Habitat for Humanity and other building organizations will use ladders in building projects. Goodwill, Salvation Army and other thrift stores also accept aluminum ladders for resale.
Call a few metal scrap dealers in your area and ask about their current rate for aluminum. Take the ladder to the place offering the highest rate. Take other aluminum items if you have them, such as aluminum cans. When you arrive, they'll assess and weigh the items and offer you an amount for them.