If Santa left an Amazon Kindle under your tree, here are some sites where you download free eBooks.
Free eBooks at Amazon: these include classics that are out of copyright, and limited-time promotional ebooks.
Jungle Search: provides a price range search at Amazon, so, for example, you can look for books less than $5.00 or even less than $1.99. Very cool!
Feedbooks: a mix of books for sale and free books from new authors, as well as public-domain classics. Of particular note is the Kindle Download Guide that is a catalog of available books. Simply download the Guide to your Kindle, and you can click your way through it, as it has links to the actual books.
ManyBooks.net: More than 29,000 free books
FreeKindleBooks.org: offers ebooks from Project Gutenberg that have been converted to either MOBI or PRC format
Here’s an article and a demo video on how to convert Google Books to Kindle. Because of the conversion step, I wouldn’t start here, since many of these books are already available in a Kindle friendly format from other sources.
And, if that’s not enough, here’s an article from ReaderReview that list 29 sources for free (and low-cost) Kindle books.