Godot Engine - Feature-packed 2D and 3D open source game engine.
CodeKeep - Codekeep lets you store and share bits of code and text with other users. Snippets can be organized into folders/labels for instant reuse.
Unity - The multiplatform game creation tools for everyone.
Anyfiddle - Build, run and share code in any language from your browser
GDevelop - GDevelop is an open-source game making software designed to be used by everyone.
CodeMyUI - Handpicked code snippets you can use in your web projects