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RPG Maker - Make your own PC game with RPG Maker. Our easy to use tools are simple enough for a child, and powerful enough for a developer. Try it free today!
Unity - The multiplatform game creation tools for everyone.
Godot Engine - Feature-packed 2D and 3D open source game engine.
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FlatRedBall - Cross-platofrm 2D game engine using C#, focused on developer productivity, transparency, scalability, and ease of use.