Project Gutenberg. Find free online books such as The Adventures of Sherlock Holmes, Pride and Prejudice, Jane Eyre, and The Adventures of Tom Sawyer, among thousands of others.
Kindle E-Books. More classics, with modern books thrown in as well. Wuthering Heights, Lord, Teach Us to Pray by Andrew Murray, and The Pursuit of God, to name a few.
Online Literature has lists sorted by author, title, and time period.
Inspired Reads. This is a blog that offers free Christian ebooks every day. The style varies, but I've found some great free resources.
$1 or Less
Ebay. I just scored a large hard cover picture book lot for under $1 each shipped, including many award winning books. I was looking for some new picture books for my preschooler, and this deal made my month! I hadn't thought to look on Ebay before this for "Lots" of books. Buying in bulk saves shipping.
Half Priced Book Store. Their clearance section often has books that range from $1 or less. Go prepared to hunt. It's not really organized. (At least ours isn't).
Garage Sales. Again, if you enjoy the hunt, this is a great way to pick up books under a dollar.
Paperback Swap. You send your books to other members, and then you get credits for books that you want. Although you're still paying on average $3/book that you ship, they give you credits in advance just for signing up, so you start out ahead of the game. At this point, though, I've been in it long enough that it feels like I'm just paying $3/book.
Amazon used books. I've found that many of their used books also qualify for Free Super Saver Shipping on orders over $25. I used this option when ordering school books this year.
Half.com This is a great resource, but your minimum shipping rate is $3.99/book, so if you add a 75 cent book, you're almost at $5. This is still much better than running out to the local bookstore, though! And this summer, I found a free shipping special, so my 75 cent books were actually only 75 cents!
What treasure am I missing out on? Where do you find your book deals?