Our local homeschool bookstore has new and used books. They also buyback books. Inside there is a clearance rack, can usually find some decent stuff depending on what you're looking for and what they have. But outside....... there's a bin labeled FREE. This is for stuff from the clearance rack that they couldn't sell, or things that other homeschoolers want to get rid of. I've found some real gems in there, as well as seen a few things that make me thing - that should have been a fire starter or recycled (books totally falling apart, workbooks that are completely filled in) so it's hit or miss, but I've seen hardback Saxon math texts in there, Abeka readers, science texts from Christian Liberty Press, some of the 'If you lived in....." books, once the bin was filled with some great read alouds in good condition like a couple of the Little House books, some Roald Dahl etc.
Online I used to like enchanted learning.com, it's well worth the $20 per year fee when you have smaller kids. We liked starfall back in the day too. I hear great things about letter of the week. I'd love to be able to afford brainpop. You can usually find some pretty amazing stuff, even full thematic units sometimes, at museum websites, and the National Park Service Jr Web Rangers is great. Coolmath.com is fun and believe it or not there are some really educational and fun games at the Disney website.
I know there's lots more, my favorites folder is sub catagorized and packed full but those are what strike me off the top of my head.
Oh - we also love our subscription to Kids Discover.
