Sell your treadmill to someone else. You can use a website such as Craigslist.org or eBay.com, or post an ad in your local newspaper. Set the price according to the condition of the treadmill.
Donate the treadmill to a charity. Most charities will accept your used equipment, regardless of condition, and you may also be able to use the donation as a tax write-off. You can also donate the treadmill to a local school or community center.
Sell the treadmill for scrap. Many scrap dealers will take your treadmill for parts and compensate you.
Give your treadmill to an e-recycling or hazardous-waste firm. Many cities offer a hazardous-waste dump where you can drop off the treadmill. Be prepared. Some cities charge a disposal fee.