Amazon launched a new shopping app on Android called Underground, which is a collection all of the free apps offered throughout the Amazon Appstore. Underground gathers and displays apps and games that are "actually free" as Amazon calls it — with no hidden costs like in-app purchases. Amazon says users of Underground have access to over $10,000 in free apps and in-app purchases through Underground that would normally cost them if purchased through Amazon's regular Appstore. But unless you are one of the hundreds of people still using the Fire Phone, it's still a pain for most people to install any apps from the Amazon Appstore on an Android device.
To keep you from paying the in-app fees some of the apps and games in Underground routinely charge, Amazon has worked out a deal with developers where it pays them on a per-minute played basis. Apps like Star Wars Rebels: Recon Missions and Angry Birds Slingshot Stella are now totally free in Amazon Underground.
The success of Amazon Underground depends entirely on the quality of the free apps it will offer
Due to Google Play Store rules, you can't download Amazon Underground through the app store, so you must head to Amazon's website if you want the new app, and jump through a few hoops to get it installed on your Android device. This has been the biggest hindrance (besides a lack of content in comparison to Google Play) to success for Amazon's Appstore — very few people want to deal with he hassle of changing security settings and installing an .apk to access content that is largely available in the Google Play Store already. Whether Amazon Underground can change that mentality will largely depend on the quality of content it offers for free in its Appstore.
Amazon also got rid of its Free App of the Day, according to Gadgtspot, which is likely related to the launch of Underground. With an entire section dedicated to promoting completely free apps, it looks like Amazon has found a nice way to drop its daily free app program.
Update: August 26th 11:40 AM: Updated with additional information from Amazon