I check this guys site every day:
http://www.fkbooksandtips.com/
He features several free books per day that are rated 4 stars or higher on kindle and usually links the free app of the day as well. He also lets you know the "kindle deal of the day" which is the deeply discounted book for the day. I rarely pay for books, just download the free ones. That way if I get a stinker, I don't care if I don't finish it.
The other free link I use is this one:
http://www.ereaderiq.com/free/
I don't care for that one as much because even though there's a ton of books right in front of you -- you have to wade through a ton of junk (and I mean junk) to find anything worth reading.
You can also just go to the search bar in amazon and search for "Free Horror books" or "Free Mystery".
Have fun... I've read more books in the year I've had my kindle than I've ever read in my life!