A Lot of people have been caught up in the Bootstrap hype. It has got to the point where nearly every website/theme was built using Bootstrap. Now that is all well and good and I use bootstrap myself on many projects but it is not the only UI framework out there. It is scary how many people think that it is.
This blog post serves as an archive of sorts to showcase my favourite web UI frameworks.
Semantic UI is an incredible framework with many feature rich components. The docs are great and the design language makes it easy to theme to your specific needs.
You denote semantic elements with a fluid and "semantic" class structure which make it really easy to learn too.
Materialize is an awesome little framework based off of Google's Material design.
It is not as feature rich as Semantic or Bootstrap but is a pleasure to use. It is particularly useful for building apps that need to look and feel like native Android apps.
Bootstrap, as previously mentioned, is obviously the most popular out there. If you are working in the web space you need to know how to use bootstrap. The great news is that their docs are great and when you get stuck there are lots of people to help you out on Stack Overflow and other
Have more that should be listed here?
Just list it in the comments and I will add it.