What UI Design frameworks are available?

Material Design

Bootstrap Well,, no need to mention this one, the best known.

Bulma: The modern CSS framework that just works.

Semantic UI: the css class names are way better and simpler than Boostrap and Material Design with the same flexibility.

Carbon Design System: A design language create by IBM with support for different Front End frameworks like vue and react

Sprout social: Although it doesn't provide resources for design it does provide helpful design guidelines.

Ant Design System: Well known design system, most resources available for React. It shines in the graph visualization design.

Tailwindcss: Can not forget to mention this framework that is atomic in philosophy, different of all other, Tailwindcss provides you with the most basic design elements, instead of components which provides you with flexibility to design your components as you wish.

Braid Design System : best known for using the stack concept for spacing.