My old hometown's system used to do a semi-regular culling at the yearly mini arts & crafts fest in the park. They'd lay out 5 long rows of tables and cover them in old books that had to be retired. You could take as much as you could carry for a donation of any size.

I used to go when I was in my teens and had two dollars to rub together, and I still go back now in my 30's if I'm in town (with more of a donation). I've found some real gems.