These are my personal favorites.
- https://getbootstrap.com - Pretty much solid UI framework
- https://webkul.github.io/micron/ - Microinteraction animations
- http://www.typenugget.com/ - Instant CSS theming with CDN
- https://wiredjs.com/ - Paper Themed Components
- https://sarcadass.github.io/granim.js/index.html - Basic Gradient Animations
- https://multiple.js.org/ - Background sharing across multiple elements
- http://parsleyjs.org/doc/examples/simple.html - Not really UI library but basically a form validation tool
- https://animejs.com/ - Cool Animations, plain and simple
- https://michalsnik.github.io/aos/ - On-scroll animations
- https://rishabhp.github.io/bideo.js/ - Full screen background video
- https://nosir.github.io/cleave.js/ - Input Formatter/Masking
- https://madeas.github.io/box-shadows/ - Box-shadows, bc why not??
- https://github.com/victorvoid/placeload.js - Placeholders, shining, and shimmering
TBD Status: Adding more to this as I get more experienced