http://postcrossing.com - A great place to go to swap postcards with other people internationally for free. Of course, you have to buy the postage to send the postcards. It's over a dollar to send to foreign countries from the USA. Postcrossing is a project that allows you to send postcards and receive postcards back from random people around the world. That's real postcards! Not electronic postcards.
Paper Back Swap. Swap PaperBack books with other members of the PaperBack Book Swap Club on line. I LOVE this place! http://www.paperbackswap.com . This used to be free. Now I think it costs about 50 cents for each swap. Or one could pay a yearly fee.
Swap a DVD - Swap DVD's with other Club members and get movies and such mailed directly to you!
Swap.com is where more than 500,000 swappers worldwide are turning what they have into what they want, with more than 1.5 million items available for swapping! I haven't used this in like forever. I'll leave the link here though.
http://www.listia.com/ - Turn clutter into credits. Credits get you things you love! I used to use this. I don't any more. Check it out if you wish.
1) Give and Get - Sell your unwanted items and get things you love in return.
2) Discover - See what you can get on Listia with Ink.
3) Earn More - Rack up more Ink for every item you sell.