Amazon has ebooks available for free download every day. They are usually new authors who make it free so that people buy the rest of the book installments. Bookbub is one of those sites that finds these free deals and either emails you the list, or if you follow their Facebook page, posts it on there. This site lists free ebooks for Kindle, Nook, Kobo and other devices as well (based on what is available). Email sign up is needed for emailed lists. Or you need to follow them on FB.
Another good site for free classic ebooks is Project Gutenberg. They have ebooks in various different formats including text.
A site that I personally use, a lot, is the Internet Archive.
Warez-bb is a forum where users post links to the various ebooks they have, based on requests. More often that not they are audio books.
Last, but not least, if you have the patience and/or are bored, the Amazon Kindle section of the website lists the top 100 free ebooks (updated hourly).
This is for fantasy:
Main Kindle ebooks page:
If you look to the right side of the page, you can see best sellers based on category. Upon clicking the 'See all best sellers' link, you can look at the 'Top 100 free' portion which lists the current top 100 ebooks being downloaded.
Addendum: I have not mentioned any bootleg sites here because, you know, bootleg.