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 syntax, but it is not Python. It's very powerful, easy to learn, and it's free of unnecessary things because it was custom built for optimized integration with the Godot Engine.
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 Easily expanded scripting system
With 3.0's addition of NativeScript and PluginScript via GDNative, developers can easily define bindings for new scripting languages. In addition to the primarily supported C++, GDScript, VisualScript, and C# languages, the community has contributed D, Nim, and Python as well with more on the way.
Cons
Con Primarily supports own proprietary language (GD Script)
While it's very accessible, and if you know Python you'll pick it up fast, having to learn a new language to fully make use of the platform can be a bit discouraging. And for those learning to code as well as learning Godot for the first time, many would rather learn a language they can 'take with them' when they explore other platforms in the future.
Con No console targets
Given that you can target both desktops and consoles with the same code base in other engines, the lack of support for consoles in Godot is pretty hard to get past if targeting desktops for a game. But asking for an open-source engine to target consoles is probably too much to ask. But it would be interesting to see some legacy consoles targeted even if current ones cannot be.