Amazon is set to release a new game engine through their site that requires no royaltys. You will be able to download everything for the engine to start developing your own games from Amazon for free.
This engine is known as Lumberyard, and has deep integration with Amazon Web Services for the server infrastructure for online play, and with Twitch, so games can be easily shared with others by streaming.
The engine is nothing to laugh at either. Lumberyard has enough features to create current-gen, triple A titles for PC, Xbox One, and PS4.
Amazon based most of their new engine off of CryEngine, an incredibly popular and expensive engine for developers.
Lumberyard is currently in beta, but you can still download it, for free, here.