http://m.gutenberg.org - Mobile version of the Gutenberg site. This one is awesome. If you go to this website using your Kindle, you can download the Kindle versions right to your Kindle with no charge or fee.
http://www.feedbooks.com - Thousands of free eBooks. Download the Kindle format. They also have a mobile version of the website which is worth checking out - click on Help at the bottom of the webpage. And you can download a catalog to transfer to your Kindle which then allows you to download eBooks instantly via your Kindle