Godot Engine is a feature-packed, cross-platform game engine to create 2D and 3D games from a unified interface. It provides a comprehensive set of common tools, so that users can focus on making games without having to reinvent the wheel.
Hi there! LÖVE is an awesome framework you can use to make 2D games in Lua. It's free, open-source, and works on Windows, Mac OS X, Linux, Android and iOS.
DragonRuby Game Toolkit is a professional grade 2D game engine. It's tiny (~3.5MB), fast, and cross-platform (including consoles and mobile).