CryENGINE is developed by Crytek. Interestingly, this game engine was initially created to produce a game demo for Nvidia, a graphics processing unit (GPU) manufacturer and because of that demo's great success, the game (Far Cry) was taken into full production and is now a commercial success. The game engine itself is also very successful.
The engine is freely available along with the full source code. There are no financial obligations for commercial use of CryENGINE such as royalties. This engine is capable of high-quality visuals and excellent performance, and you can develop games for the following platforms:
- Linux PC
- Oculus Rift
- Playstation 4
- Windows PC
- Xbox One
For further information on CryENGINE, I recommend Mastering CryENGINE by Packt Publishing: https://www.packtpub.com/game-development/mastering-cryengine.