The latest stable release is also available on Steam.
Godot's full source code is provided to you under the very permissive terms of the MIT License. No royalties or payments are required, you have full control of your game and engine.
Godot development is open. This means you can fix or improve any part of the engine yourself and choose whether to contribute it back to keep it private. New features are always available to use and test, without the need of having to wait for the next major release. Compiling Godot from source is very easy (it has no common dependencies) and the process (for each platform) is well-documented on the wiki. To obtain the source code (and consulting the documentation), please visit the GitHub project page.