Call your local library. Many libraries accept homeschool curriculum materials. These are made available as references for homeschooling parents. Parents can make copies of lessons or compare curricula to decide which one they want to use with their child.
Let your local homeschool organization or co-op know that you have books you no longer need. Let them know what grade level the books are for and leave your contact number in case anyone is interested in them. This is a great way to make sure someone who will use the book will get it.
Post your books on an online swap site. There are plenty of homeschool curriculum swap sites that would allow you to offer your book to anyone who needs it free of charge.
Use Freecycle. Freecycle is a group set up in Yahoo groups. They allow you to offer items for free to anyone that wants it. The groups are set up by location. Find a location near your own hometown and list your books. Other members will read your post and contact you if they need your particular book. You can then make arrangements to get the books to them.
Call used book stores in your area. Many of them will gladly accept donations. This allows them to offer the book at lower cost for someone who couldn't afford to pay full price.