ViniSketch Designer is a free application -for Mac OSX- for designing and creating mobile applications using native and web technologies.
It comes with feature-rich visual editors for designing the GUI + dataflow and code editors for keeping the development under control.
The engine of the app has support for multiple platforms and you can managing your mobile app for each device you're targeting.
ViniSketch Designer is currently compatible for building iOS + Android apps, however, compatibility for other popular platforms are under development.
It can auto-generate Phonegap projects and there is a built-in HTML5 simulator + debug tools based on WebKit WebInspector.