The Mahjong game in the recent freebies is
great and one I actually had already paid for (Google credit from their Reward app, I think). I was excited to see Threes, though I'd recently picked it up in a Humble Bundle. I like to have backup download options, so I got it anyway.
The company that makes the Mahjong game (G5 Entertainment) and the Abyss game gives away or hugely discounts games regularly, across all operating systems and in both Google's Play and Amazon's App Store. I follow their Facebook page for notifications, but if you visit their website they update it when they update the sale. Their two free titles in this Amazon sale still have a sign up, actually.
http://www.g5e.com/sale