Recs.
Updated
Godot is a fully featured, open source, MIT licensed, game engine. It focuses on having great tools, and a visual oriented workflow that can export to PC, Mobile and Web platforms with no hassle. The editor, language and APIs are feature rich, yet simple to learn, allowing you to become productive in a matter of hours.
SpecsUpdate
Pros
Pro Free, open source and cross-platform
Godot is under MIT license so you can use it for your project without paying a single buck to its owner. Godot is licensed under MIT license. Anyone can grab the source from github.com/okamstudio/godot, and compile the engine him/herself. It targets Win/Mac/Linux/Android/iOS/BB10/HTML5.
Pro Internationalization of the editor
You can change the language shown in menus. Godot translations: https://hosted.weblate.org/projects/godot-engine/godot/
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.
Cons
Con Difficult to optimize
Godot has an OOP architecture. Everything is an object internally and data is spread among many classes, thus it's difficult to optimize (i.e. not cache friendly, difficuly to vectorize or paralellize, etc).
Read about "Data Oriented Design" for more info about the problems and solutions.
Con Self-designed scripting language
Implementing new scripting language instead of using an existing scripting engine (such as Lua or V8 JS) looks expensive, but easy to learn.
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.