All the above CSS frameworks are open source. And they are widely popular in the community. Other than these there are multiple frameworks available. But I have not used them that much. But here you go,
Material Design Kit
UI Kit
BaseCSS
Foundation
Primer
Skeleton
I found this link on GitHub, consisting of various frameworks. It's really good.
Top comments (4)
Bulma is amazing and easy to use. Also 0% JavaScript
+1, it is really awesome and easy to use with React.
My personal favourite CSS library is Bootstrap. Apart from that, I have worked on a few more CSS frameworks.
Tachyons - link
Semantic UI - link
Pure - link
All the above CSS frameworks are open source. And they are widely popular in the community. Other than these there are multiple frameworks available. But I have not used them that much. But here you go,
I found this link on GitHub, consisting of various frameworks. It's really good.
I hope my answer helps.
I'll admit to falling for their GitHub proof of concept