Bevy

Bevy is a game engine written in Rust and built around an entity component system (ECS): you build game objects by snapping data components together instead of inheriting from classes. It renders 2D and 3D, targets Windows, macOS, Linux, the web, iOS and Android, and it's free under your choice of MIT or Apache 2.0. No royalties, no seats, no revenue share.

There's nothing to host. Add it with Cargo and write code. Hot reloading covers scenes, textures and meshes; gameplay changes still mean a recompile. A nonprofit foundation pays maintainers out of donations, and version 0.20 shipped in early October.

Solo Rust developers and small code-first teams: free, and a real option. Medium and large studios that need an editor and a stable API: Godot or Unreal for now. Unreal is free until $1M in lifetime gross revenue per product, then takes 5%.

The catch: Bevy is pre-1.0 and says so. A release with breaking API changes lands about every three months, and migrations aren't always easy. There's still no visual editor. Recent releases ship editor building blocks (UI widgets, a scene format, a remote inspector in progress), not the editor itself. Everything is code.