Godot Engine - Feature-packed 2D and 3D open source game engine.
Pygame - Pygame is a set of Python modules designed for writing games.
GDevelop - GDevelop is an open-source game making software designed to be used by everyone.
GeeXLab - GeeXLab - cross-platform tool for game development, 3D programming, creative coding and prototyping
MonoGame - MonoGame is an open source implementation of the Microsoft XNA 4 Framework.
LOVE 2D - Hi there! LÖVE is an *awesome* framework you can use to make 2D games in Lua.