Defold is a free lightweight 2D game engine from King, available at https://www.defold.com.

Defold provides you with everything you need to make a game. It has a full-featured editor which offers tools for creating UI, animation, authoring physics, designing levels, and much more. The engine offers fast, live iteration through code hot reloading. Hot reloading code means whenever you make a code change, it can be hot loaded without having to restart the game. Defold offers a large number of tutorials for learning the engine; these tutorials are online at https://www.defold.com/tutorials/getting-started/.