While the Amazon Underground program was in existence, it offered free Android apps that were labeled as "Actually Free." These apps included paid games that could be downloaded for free and free-to-play games that disabled in-app purchases and included some in-game items for players.
You can play some great games for free with Amazon Prime
This game is probably the best Sonic game ever made after maybe "Sonic 3", and is definitely worth checking out. The time travel component of the game is very well done, as it lets you explore different levels and paths, some of which are hidden in different time periods. Additionally, you have to destroy objects in the past to ensure a good future.
There are four different themes and layouts for each level, with variations of past, present, bad future, and good future. It's an impressive feat for a game from 1993, and the port has done a great job two decades later of adding small tweaks and refinements, along with features like widescreen aspect ratio, both Japanese and North American soundtracks, and mobile controller support, to make this the definitive version of the game.
Requires Android 4.2 or later.
