Paperbackswap.com is an amazing site. Curriculum is SO expensive and I've been able to get some great finds off of there. Here's how it works: You list books you want to trade. When someone requests your book, you mail it to them. When they receive it, you get a credit to request a book for yourself! There is over 5 million books listed and just in case they don't have something you want, you can put it on your "wish list" and they will notify you when it becomes available.
Money is really tight at our house so this has been such a blessing. I was able to find 6 years worth of spelling (normally about $10 a book), reading comprehension for Mason, and Nate's algebra books. The algebra set is $85 new!
They have much more than homeschool curriculum. In fact, that is just a small portion of what you can find. If you decide to sign up, put my name down as a reference and I will get a free credit. It's definitely worth a look :)