There are lots of ways to unload items you aren't using without sending them to the landfill. You could sell things on eBay or Craigslist, donate to your local Goodwill or Salvation Army, or give them away on Freecycle. A new UK service puts a charitable twist on the donation model: JumbleAID lets you use unwanted goods to raise money for the charity of your choice.
The service lists over 25,000 charitable organizations, screened by Charities Trust, that you can raise money for. Once you've selected your charity of choice, you just list your item or items along with photos and a description. Set a target price, and you're ready to raise funds!
JumbleAID is a totally free service, which means they don't keep any of the money that you raise. They handle donations through a third party, Charities Trust, who does take a 4% cut for administrative fees.
I'd love to see something like this for the U.S.! Maybe even in the form of more localized services, so folks wouldn't have to ship their goods all over the place. What do you think?