Billing themselves as "A sustainable exchange platform for a clutter-free world," Swapabee is an online platform (and/or app, if you prefer) based on the idea that one person's junk is another person's treasure. Sort of like what the original eBay was, but without the bidding and the money.
How it works: You take whatever you've got lying around and don't want, figure out a rough value, then post a photo and price range to the site. You can then either search for items you want of the same value, or you can let their algorithm bring up five available items you might want to trade it for. You and your fellow trader then chat through the platform to decide how to exchange (mail or in-person).
(Note: You are not limited to swapping items within the same categories, as the images above might lead you to believe.)
Swapabee is a nonprofit that nevertheless has operating costs, so while the first two swaps are free, each one after that costs 50 pence. Yes, that's right, "pence;" as it stands, the platform is UK-only.