I love Park City's annual book sale at their library (pictured in the background - formerly the original PC High School).
$5 for a canvas bag - stuff it full of as much as you can! So we loaded up. And I love not having to worry about return dates, keeping track of it all, preventing damage, etc. Thanks to sales like this (and Nickel Auctions, yard sales, etc) I never ever borrow kids books from the library.
It also gave the kids something to do while sitting waiting for the Park City Miner's Day parade to start.