There's a phenomenon sweeping my bookshelves. It involves things I have bought, read, in the most part enjoyed, but don't feel the need to keep forever.
One of the customers in the bookshop, Annabel, told me about read it swap it. For the cost of postage, you can swap books you have finished with and read new ones.
You list some books, someone wants one, so you look at their list and if there is something you fancy reading, you swap and send each other the books. There's a ratings system similar to ebay to keep track of swapper reliability.
It's cheap, it's green and in my experience the site is well run and the whole experience, pleasant. It is, though, only available to folks in the UK (I assume to keep postage costs low). As far as I can see, there isn't really any other down side, apart from the failure to reduce my book mountain. And don't blame me if you get lost browsing their library...