Perhaps, you will find not so widely known free e-book library www.bookyards.com a valuable source in your search. They proclaim themselves as a “library to the world”, but against the grand proclamation, it has a modest collection of approximately 17, 000 downloadable e-books. It does not require any forms of registration for downloading e-books. 
And all the e-books are available in supported by Kindle .PDF format.
You can enjoy Grapes of Wrath by Steinbeck for free (compare to Amazon‘s $12.99); or Hemingway’s The Sun Always Rises ($11.99 on Amazon); F. Scott Fitzgerald’s The Great Gatsby ($10.99 on Amazon). However, I have to admit, Bookyards’ fiction section is quite limited.
This free e-book library is mostly education oriented. So, a student, or a researcher, or just a curious erudite might find this resource a very useful one. If you were looking for an electronic version of C. Bresciani Turroni’s Economic Policy For The Thinking Man, or perhaps the Marquis de Nadaillac’s Manners And Monuments Of Prehistoric Peoples, or Charles Marquis Smith’s Electric and Magnetic Measurements (all of which are unavailable in Kindle edition on Amazon) – then Bookyards will become your favorite source for e-books.
The organization of the site is pretty easy to navigate; all the available e-books are a couple clicks away from downloading. Also, Bookyards displays the list of the most popular downloaded e-books on the front page. To my giggly astonishment, one of the most downloaded books is not Adventures of Huckleberry Finn or The Kama Sutra of Vatsyayana (kudos to Project Gutenberg’s Top 100), but it is Sharipov’s Classical Electrodynamics! See? What an extremely different contingent of e-book users frequents Bookyards.
I hope you will find this library useful in your free e-book search.
Great information as always. Thank you!
Unfortunately, this site lists books that are still very much in copyright, and furthermore has links to even more stolen books. You really should do your research before you start listing sites that are little more than book theft.
As an author, I am completely opposed to DRM in any form, and believe my readers should be able to use their purchase in any personal way they choose. But the key word there is purchase.
You guys might be interested in some free Kindle books on Amazon from Kaplan Publishing – they’re available until January 17th 2011.
Here’s a URL for them:
http://www.amazon.com/gp/feature.html/ref=br_lf_m_1000649891_grlink_1?ie=UTF8&plgroup=1&docId=1000649891
I’m all for free books but, just as one example, John Steinbeck only died in 1968 so his books are all very much still in copyright. Offering them for free is copyright theft, and I can’t imagine this site surviving if it continues like this.
My interest focuses on finding libraries with free e-books available. I do not focus on the copyright issue, because that would eliminate listing 99.99% percent of the free e-book libraries. This is a source for free e-books and not all of them are copyrighted. Whether to download a copyrighted book or not – is on each and everybody’s conscience. I’m not into reiterating basic postulates, “stealing is bad, please, do not steal”. It would be more appropriate for me to teach that to my future children.