OrbTk is described as 'The Orbital Widget Toolkit is a multi platform (G)UI toolkit for building scalable user interfaces with the programming language Rust. It's based on the Entity Component System Pattern and provides a functional-reactive like API' and is an app in the Development category. There are four alternatives to OrbTk for Linux, Mac and Windows. The best alternative is WebRender, which is both free and Open Source. Other great apps like OrbTk are Conrod (Free, Open Source), Sauron native (Free, Open Source) and Azul.rs (Free, Open Source).