Figma's Official Design System

Figma in their latest update included their own Design System called the 'Simple Design System', not exactly the greatest of names but it's still a solid set of components.

It's worth noting it's a surprisingly small set of components, but they've all been well crafted, carefully structured and with additional comments. The examples page is also fantastic as it provides a nice set of wireframes that you can easily use to create some clean and professional designs, they also provide great inspiration.

Also something that's a little different is the composition guide page, that provides recommended layouts for desktop, tablet and mobile, plus recommended form layouts and menu layouts, it really is just a great set of simple components.