I take a look at this site every couple days. Well written, descriptions lifted off the
Amazon page. She also lists Nook and other sites for free pdf file downloads, too. And if something needs to be converted, she explains it.
http://booksontheknob.blogspot.com/
Free downloads, especially at Amazon, don't last more than a couple days -- unless they are those free public domain snoozers.
You pretty much have to snap them up as you find them. The "free" part is part of a promotion. Once the promo is over, the books go up to full price again.
Kindle downloads are free because Amazon makes them free
at the Amazon site. I don't think other sites can list a Kindle book for download, that isn't going to be directly downloaded from Amazon. I'd be wary of those kinds of downloads for viruses, and also the DRM licensing may be funny, unless they are from some legitimate site.
Now, there is the mobipocket format that does work on Kindles, but are not necessarily from Amazon. And yes, those can and be downloaded from legitimate, safe sites.