Amazon offers a lot of Kindle books for free on their site, even excluding the public domain ones. eReaderIQ is an easy-to-browse database of all of Amazon's free offerings at any given time, and it's updated hourly — meaning you have a large, accurate list to browse for completely free reading material. You don't need to sign up or anything, you can just head over and start browsing. Clicking on a book will take you to its Amazon page, from which you can send it right to your Kindle (or other device running Kindle software).
eReaderIQ also has a neat alerts feature, where you can track other books on Amazon and get alerted to when their price drops. You can import your Amazon.com wish list, or just paste in its URL on the price tracker. Then, just give it your email and the price drop you'd like to be notified for (e.g. entering $US1 would alert you when the book's price drops by $US1). It's pretty handy when a book is a bit more expensive than you'd like, or when it's actually more expensive than the physical copy. Hit the link to check it out.