Last year, I reviewed Homeschool Library Builder. In a nutshell, this site is an online bookstore that specializes in low-cost, homeschool-friendly books that making educating your kids that much easier.
Prices are low, and I do mean low. Most items are 50% or more off the retail price, with many bottoming out at what I consider yard sale prices--$1 a book. The site is easy to use, memberships are free, and, best of all, the books are sorted into logical categories. Two homeschool moms run this site, and it's obvious: the selections are well defined, and there is surprisingly little "fluff" rounding out the offerings.
The downside, however, is that when I started typing in my own personal wish list, I couldn't find a single item I was hoping for. Perhaps because my shelves are already crowded with the basics, I came away empty-handed. Paperbackswap treats me the same way, though, so I didn't take it personally.
I do think that this is a wonderful, free resource that all homeschoolers should keep bookmarked. You never know when you'll make a true find!