One of my favourite authors, Leighann Dobbs, has set up a website where she lets you know about the best free and bargain cozy mysteries for that week. I think this is a great idea and I've just signed up to her newsletter.
Here's the link if you're interested:
http://bestcozymysteries.com/best-free-cozy-mysteries/
Here are some of the books listed so far:
You'll find more on Leighann's website along with the store links: