Slydris, originally $1.99, is today's free app. A supposed twist on Tetris, where you slide the lines across the screen to make matches and eliminate blocks. From the reviews I read, it's a good thing the game is free. Frustrating for some. Give it a shot.
Personally, I like to keep everything in one place for simplicity. Even though Amazon does have advantages (more security and scans, so articles have said), Google Play is the centerpiece of the Android system in many respects. I'm content. Can't switch.
For those who do, Kindle Buffet: Kindle Buffet
keeps track not only of free ebooks (for which they are more known) but also the latest free-but-normally-for-pay apps offered.