The best alternative to Unity is Panda3D, which is both free and Open Source. If that doesn't suit you, our users have ranked more than 100 alternatives to Unity and six of them are so hopefully you can find a suitable replacement. Other interesting alternatives to Unity are Phaser, CrossBrowdy, AppGameKit Classic and Whitestorm.js.
Panda3D is a game engine, a framework for 3D rendering and game development for Python and C++ programs. Panda3D is Open Source and free for any purpose, including commercial ventures, thanks to its liberal license. Go ahead and grab it here.
The renderer is very bad and outdated. The development of the engine is very low.
Phaser is a fun, free and fast 2D game framework for making HTML5 games for desktop and mobile web browsers, supporting Canvas and WebGL rendering.
Multimedia JavaScript framework to create real cross-platform and hybrid game engines, games, emulators, multimedia libraries and apps.
AppGameKit is a simple cross platform game development tool that runs on Windows, Linux, MacOS and Raspberry Pi. A version to run on Android and iOS is also available. With AppGameKit you can use a simple Basic like programming language to develop cross platform applications, 2D...
Whitestorm.js is a framework for 3D web apps built with Three.js technology. It implements a core for fast development of a 3D scene with physics.
AGKSharp is a free implementation of AppGameKit to be used with C# and .NET.
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Panda3D is open-source and has excellent support for scripting in Python, making it a great choice for rapid prototyping.