I’ve used Bootstrap for a lot of my projects, but recently I’ve been interested in Materialize CSS. It offers a lot of similar components compared to Bootstrap, but with a cleaner syntax and a design style based on Google’s Material Design.
It’s still a work in progress, but definitely something to keep on your radar if you’re looking for an improvement over Bootstrap, or just something different.
If you’re working with EmberJS, there’s a great addon called ember-cli-materialize that provides a lot of the Materialize CSS functionality in components. It’s definitely a large library, but can be useful if you’re using a lot of the Materialize pieces.