Sid posted about Book Mooch some time ago and I recently got around to joining. So far, so good. You get points for uploading books that you're willing to give away, then can use those points to mooch books from others in the group. So far I've mooched one and been mooched for one, but I sent two more requests today. I had quite a few extra Louis L'Amour books because I inherited a collection from a friend's Uncle and ended up with quite a few duplicate copies. I've picked up various duplicates over time, as well. The only cost is postage to mail books that people mooch from you. Seems like a pretty cool idea, especially since many of the books I want are actually older ones that aren't on the shelves at the local bookstores.
For more on Biblioholism, check out Sphinx Ink's post today.