Now UI Kit is a responsive Bootstrap 4 kit provided for free by Invision and Creative Tim. It is a beautiful cross-platform UI kit featuring over 50 elements and 3 templates.
Framer X, a powerful new product that seamlessly blends design and development. Coming Fall 2018.
Shards UI Kit is a free, modern and lightweight UI toolkit based on Bootstrap 4 that comes packed with 2 extra pre-built landing pages and 11 custom components that you can use to kick-off your next project.
Material Dashboard is a Free Bootstrap 4 Admin built on top of Material Kit (https://demos.creative-tim.com/material-kit/index.html). It is Open Source released under MIT License so you can use it for personal and commercial projects without any issue.
Startup and SaaS Business Focused UI Kit based on Bootstrap 4 for quality and faster UI development. Ayro UI comes with 400+ UI elements, 150+ sections, essential pages and all you need to build full-featured user interfaces.
Gridbox is a simplest visual interface builder for css frameworks. Supports Bootstrap 3, Bootstrap 4, Foundation 6