Recs.
Updated
Godot is a free, open-source, and MIT licensed game development environment for 2D and 3D games.
SpecsUpdate
Pros
Pro Easy to learn scripting language
Godot has their own scripting language called GDscript. The scripting language is easy to learn with python-like format, but it is not python. More like a mix of Javascript, PHP, C++. It's very powerful, easy to learn, and it's free of unnecessary things because it's designed for this purpose.
It can be used to add custom behaviors to any object by extending it with scripting, using the built-in editor with syntax highlighting and code completion.
A built-in debugger with breakpoints and stepping can be used and graphs for possible bottlenecks can be checked.
Pro Free and open source
Godot is licensed under MIT license. Anyone can grab the source from https://github.com/godotengine/godot, and compile the engine themselves.
Pro Under constant development
This engine barely released one year ago has more than 1000 forks on github and about 100 developers. Not only that just a bit of browsing trough issues you will quickly find out the dev community loves new esp free technology and does not shy away from completely rewriting parts of the engine. The audio engine is being completely rewritten to use threads and so forth.
Cons
Con Lacking documentation
The documentation is certainly incomplete. It has very few examples of real application and even fewer design guidelines about how to program a game in the engine.