I've been taking part in the UKarma group on Ravelry which is a brilliant idea. You offer up something you don't want, and somebody claims it. They then have to offer up another item in response for somebody else to claim. If you offer up an item without claiming something first, you get a nice point, which you can redeem against any offer that is made.
I offered up a cone of sparkly blue "yarn" (I say "yarn" because it wasn't really yarn, but I have no other way of describing it. I then used my nice point to get myself some fibre someone else was offering.