Most of these have been mentioned before, but there's at least a few that haven't, at least the specialized ones, so I thought I'd post these lists here for your perusal. :D
Top 10 Book Swapping SitesTop 10 Places to Get Free Books (Part 1)Top 10 Places to Get Free Books (Part 2)Unfortunately, they tend to be US-centric in that even though I know some are International, the articles don't specify. :whip: