What is Onsen UI?
Onsen UI helps you develop both hybrid and web apps. If developing hybrid apps, you can use it with the Cordova / PhoneGap command line, or with Monaca tools (CLI, Monaca IDE - cloud-based IDE for Cordova, Localkit - desktop GUI).
Onsen UI is a tool in the Cross-Platform Mobile Development category of a tech stack.
Why developers like Onsen UI
Hybrid mobile
Works with any JavaScript framework
Allows for rapid prototyping
Free