From a web designer's standpoint, you can never have enough good UI toolkits in your arsenal. If you're looking for something a little different - something that will get you noticed, you should check out Shards.
Based on Bootstrap 4, Shards was created by designrevision and is a free library for creating modern-looking UIs. It has the following features:
In addition to Bootstrap, Shards is dependent on JQuery for custom components, and some Bootstrap components will require popper.js.
If you find any of this confusing, don't worry: Shards has a starter html template on their website that will guide you on how to reference all necessary dependencies.
Without question, Shards is a UI toolkit that all web designers should try. Its potential is unlimited.