This is a specialized site that is focused on just free ebooks in the Kindle Store. That makes it very different from Inkmesh, the ebook search engine, or eReaderIQ, the ebook deals site, or most other ebook deals sites.
But with the tight focus comes new opportunities for more careful searches. Freebook Sifter can show you free ebooks from just a single category or subcategory in the Kindle Store. Science Fiction, for example:
The site currently lists around 5 to 10 thousand free ebooks, including 2700 nonfiction titles. It sifts through the Kindle Store everyday, adding newly free titles and removing expired ones. 291 titles were added today.
Freebook Sifter doesn't seem to be using an affiliate code in the links to the Kindle Store, and that means they are not being tracked by Amazon. That's probably a good thing. Earlier this year Amazon changed their rules in order to discourage websites that only promoted free ebooks. Amazon made the sites responsible for the free ebooks downloaded via each site. If the ratio between free and paid ebooks shifted too much in a given month then Amazon promised to withhold any affiliate commissions for that month.
I would hazard a guess that Freebook Sifter is not using an affiliate tag because there is no money to be made - not any more.