Here at HubSpot we have built a bunch of libraries on top of Tether, both because we wanted Tether-performance, and because we saw opportunities to improve on what was available in the client-side ecosystem.
Select is a replacement for native browser select elements that is fully stylable.
Shepherd is a library for making tours of your app to help onboard users and show off new features.
A simple, easy-to-use implementation of tooltips that works well.
Where Tether does general-purpose positioning, Drop assumes that you are interested in making something which pops up next to something the user clicks or hovers on.
If you're building something that fits that pattern, Drop can make things a little easier.
A simple and reusable datepicker component for React
An Ember.js-friendly interface for tether.
If you have a cool open-source library built on Tether, PR this doc.