There are tens of thousands of free e-books on Amazon that you can download to your Kindle (or to the free Kindle app on your computer, if you prefer). Finding those free books can be tricky, but a new web site called Free Book Sifter (www.freebooksifter.com) automates the process and updates its listings regularly.
Just browse to the site, choose the book you want, and follow the link to its page on Amazon. You can then download the book to the Kindle.
In my tests, a couple of the books I clicked on turned out not to be free. Presumably because Amazon has changed the pricing since the Sifter last updated its pages. But the site is still an incredibly useful resource for book lovers who want free books.