ViniSketch Designer – Free Tool For Building Mobile Apps (Mac-Only)

ViniSketch Designer  is a free software for visually creating mobile applications using web and native technologies.

It only works with Mac OS X and provides visual editors for designing the GUI + dataflow besides a code editor to customize the stuff in detail.

Vinisketch Designer

There is a built-in simulator and debugger which is based on the WebKit's WebInspector.

The apps created are compatible with iOS + Android and other platforms are under development.

Also, if you will be using PhoneGap for the app created, it can generate the project automatically.