Read It Swap It. I have decided that you probably just don't know about it, and so I am going to (oh benevolent one that I am!) tell you about it in the hope that you will save so much money you'll feel a similarly altruistic urge to buy me a pint in the pub next time you see me.
It's quite a simple premise: you sign up, list books you want shot of (there's a helpful thing where you just type in the ISBN) and off you go. You can search for a book you'd like to read, and if somebody has it, you can request it from them. They can then have a look at your list, and if there's something they fancy they select it. Once the swap is confirmed you then exchange addresses and off you go.
Not everybody will swap with you, as it obviously depends if you've listed a book they want to read. So the more books you list, the better. There's also a star rating system, like eBay, so slack posters beware. But basically, if you can get to a post office, it's a genius premise. A lot of the hardcore members use it like a library, so all the hot new fiction is always up for grabs.
I've used the site for a couple of years now and I've never had any problems with it. Most users are lovely -I left an old birthday card from my Grandma in a book I swapped once (had been using as a bookmark at some point years ago) and the chap I sent it to very kindly sent it back to me. You get cute little thanks-for-swapping cards sometimes too. Not from me though, because I'm a miserable cow.
I like the idea of my books going off on adventures, and sometimes wonder where they end up....maybe they're in a big home library with loads of buddies, maybe they got read on a beach in Thailand and are now reclining on hostel bookshelf, happily greasy with suncream. It's also quite the geek-buzz when somebody requests one of your cult reads, then you get to check out their swap list and it's AWESOME. And then you sort of want to be friends. But you don't do anything about it, because that would be stalking.
Anyway, maybe you've got a Kindle now, maybe you're happier with charity shop rummaging and libraries or, er, actually giving the publishers and bookshops your business and keeping the whole industry afloat (I do enjoy this as well, I promise). But now you have another potential weapon in your book-gathering armoury. You're most welcome.
Happy swapping, nerdlings!