I really recommend taking a look at the HubSpot FOSS projects. They are mostly MIT licensed and basically stuff you need every day. Their libraries include
Pace, an automatic, dynamic progress bar for the top of the page (just like the one here on Hashnode)
Tether, a positioning engine for elements
Vex, a dialog library
Shepherd, for those tutorial step-by-step guides
etc.
They do not only have JS libraries, but also stuff for Python and Java.