Onsen UI - Cross-Platform HTML5 Hybrid App Framework
It is open source and designed to deliver native look and feel. Apps are made out of simple HTML tags, thus providing a gentle learning curve.
Some other frameworks are supported by community packages (i.e. not tested or implemented by the core team): Aurelia, EmberJS.
Both iOS flat and Android's Material design are included. The components are optionally auto-styled based on the platform, which makes it possible to support both iOS and Android with the same source code.