Freecycle. I would go as far as to say I'm an addict. Ever since we bought the house, things have cropped up that we need that we just haven't budgeted for, and even where we have planned for something, everything costs more than you think it will. I must admit, I was quite sceptical when I first heard about a website where you could get stuff for free and there was seemingly no catch. BUT that is exactly why I think it's so great, there really is no catch. Not only can you get some great things, but you can give your old things a home and someone will come and pick the items up from you so you don't even have to leave the house. Since we moved in, (among other things) we've got material for my crafting, a freezer, rockery pebbles, plant pots, top soil, roof tiles, and best of all, the decking in the picture above. All completely free. The decking would be worth around £1000 to buy new, and all we had to pay was the cost of hiring a van for the afternoon to shift it all (£35).
I also really like the idea that something I can no longer use can go to someone who will really appreciate it, I love hearing what people intend to do with the stuff I give away and this is a much more personal way to recycle things that giving them to a charity shop (although obviously that is still a good thing to do).
Without Freecycle, there are lots of jobs around the house and garden that we wouldn't have been able to afford to do for several years - what freebies could you not live without?