Amazon has released its very own engine, Lumberyard, which is completely free of subscription and royalty fees.
Lumberyard is based upon the foundation of Crytek's CryEngine, and can be used to create triple-A titles for PC, PS4 and Xbox One. Amazon has licensed official tools for the latter consoles from Sony and Microsoft. Mobile compatibility will reportedly be added in the future, and Amazon is currently testing support for the Oculus SDK internally.
Lumberyard will generate revenue through the use of Amazon's Web Services server technology, which comes pre-built into the tool.
Develop has more details.