Hook up your Kindle to the computer and go to this website:
http://manybooks.net/categories/
Thousands of free books (all public domain).
Also:
http://www.gutenberg.org
Also:
http://mobipocket.com/freebooks
How about free audiobooks?
http://librivox.org
When you download, use the file called MOBI or PRC format (each book has different formats for different ebook readers; Kindle uses the two noted)
Also, using your Kindle (no computer) go to the Kindle store, then select "Kindle Best Sellers". Many of these 'best selling' books are free (hence the reason they are a best seller!). These are more current books, often being offered by an author as an inducement to buy some of his or her other books (hoping that you like this free one, and so will buy the more recent offerings).
Finally, there is a book you can buy called "Kindle shortcuts, Hidden features" that not only has the foregoing information (as well as a lot of 'kindle friendly' websites), but has a link to mobipockets that you can click on, using your Kindle only, and so go directly to said website via whispernet (i.e., no need to hook up to the computer). I guess that you can go to the 'experimental' part of Kindle (i.e., internet), type in the mobipocket address, and also get to it that way.
Hope this helps.