Cloudcraft uses Material UI
Material UI provides Cloudcraft.co with a clean, professional looking and very easy to use set of UI components build with React. The few issues we've reported to the developers have been quickly fixed each time. I highly recommend using Material UI for both consumer and enterprise web apps. The styling system in particular is very nice to work with, and allows you to easily add your own brand's look and feel throughout the UI.
Kurzor, s.r.o. uses Material UI
We like the pure simplicity of Google's Material UI. It is simply too much overhead today to design custom UI styles.
GrowSumo uses Foundation for Apps
We use FoA for the grid and as the base styles for some of our components (e.g. buttons, cards).
jhabdas uses Material UI
I'm using MUI to build out the admin portion of the site under Firebase authentication.