Eva is a free and Open Source Design System for Sketch released by the awesome Akveo team. It includes 650+ symbols, 258 layer styles, 480+ icons and tons of components.
Eva Design System is a great resource to get started with a consistent design project. It is very easy to customize and much more than just a UI kit: it is structured in a way that you can easily drop in your colors, typography, and symbols to update everywhere have a clean sketch workstation.
Last but not least, Eva comes with Angular and React Native UI components libraries. A dream for any front-end developer!