D2Admin is a fully open source and free back-end product front-end integration solution. It's using the latest front-end technology stack.
Airframe React is a free open source high quality dashboard / admin / analytics template based on Bootstrap 4 & React. It works great on any smartphone, tablet or desktop.
Also jQuery version is available.
Tabler is a premium, free and open source dashboard template with responsive and high quality UI. Tabler is built on Bootstrap.