NativeBase is described as 'Essential cross-platform UI components for React Native'. There are seven alternatives to NativeBase for a variety of platforms, including React Native, Self-Hosted solutions, Android, iPhone and Mac. The best alternative is Ignite CLI, which is both free and Open Source. Other great apps like NativeBase are Shoutem UI toolkit (Free, Open Source), React Native Starter (Freemium, Open Source), React Native Elements (Free, Open Source) and Galio (Free, Open Source).
Galio is a beautifully designed, free and open source React Native UI built from the ground up as a framework. Carefully crafted by developers for developers. Ready-made components, typography, and a gorgeous base theme that is easily adaptable to each project.