cocos-engine

Cocos Creator is a free game engine with a visual editor, scene graph, and cross-platform export to iOS, Android, web, and desktop.

Cocos has been around since 2010 (originally Cocos2d-x) and is huge in Asia, particularly China. The engine handles rendering, physics, animation, UI, and audio. The editor is TypeScript-based, which is unusual for game engines but means web developers can jump in without learning C# or C++. It also exports to native platforms via C++.

Completely free. The license situation is a bit unusual (custom license, not a standard OSI one), but there are no royalties, no revenue caps, and no paid tiers. Solo indie developers and small studios targeting mobile will find this compelling. The 2D workflow is particularly strong. For 3D, it's improving but still behind Unity and Godot in tooling maturity.

The catch: English documentation is spotty. The community is predominantly Chinese-speaking. If you don't read Mandarin, you'll miss a lot of resources and forum help.