Godot - Advanced, feature-packed, multi-platform 2D and 3D open source game engine

This is a lot like an open source Unity. It is trying to be pretty comprehensive, and while a little thin in areas, they are doing a great job– a lot of value for the “cost” (which is namely, learning the new platform.)

The 2.x series is current, but 3.0 which is in the final stages is a big upgrade, with physically based rendering, a lot better shadow system, support for multiple programming/scripting languages and a visual programming language (for designers and artists to use, though its fully capable large scripts are harder to do visually.)

Atomic

Unity3d