The VMware Design group is offering its Clarity Design system as an open source project. It has a robust (free) Sketch app UI template and Library system as part of the offering.

Project Clarity is an open source design system that brings together UX guidelines, an HTML/CSS framework, and Angular components. Clarity is for both designers and developers.

Clarity’s designs stem from continuous exploration and research. These designs are built into our HTML/CSS components, which can be used any web UI, regardless of the underlying JavaScript framework. Clarity also offers a set of well-designed and implemented data-bound components built on top of Angular, one of the most popular JavaScript frameworks in the industry.

Clarity UI Template