There aren't a lot of reasons to use the Amazon Appstore over Google Play, but one key exception is the free app of the day. Until now, Google hasn't offered any similar type of promotion, but that seems to be changing with a quiet new promotion for a "free app of the week".
As Android Police points out, this promotion is currently buried under the Family tag, so if you weren't browsing in just the right place you might have missed it. Currently, the free app is a kids game called Daniel Tiger Grr-ific Feelings. Presumably this will change each week, though it's unclear right now how you can find out when a new free app is available.
However, even if you don't care about a relatively unknown kids game, it's still notable as Google has not previously had a mechanism to give games away for free. Before this, if a developer of a paid app changed their price to free, the Play Store would not allow them to change it back to a paid app. This has made giveaways like this one impossible.
While Google hasn't announced any official changes to the policy, this deal seems to work by automatically applying a discount code to reduce the cost of the app to zero. Regardless of how it works, it seems Google plans keep offering featured deals that are worth keeping an eye on.