Developers have a chance to license Unigine engine with up to 40% discount before August, 31. Since Unigine becomes even more advanced technology, prices will be higher soon. More details on licensing is available here.
The Unigine engine has powerful 3D render, integrated physics, GUI, built-in scripting system, sound module and visual tools (all of this is available both under Linux and MS Windows). It is already being used in the development of different games and non-game VR projects.