Xamarin - Create iOS, Android and Mac apps in C#
wxWidgets - wxWidgets: Cross-Platform GUI Library
Xcode - Xcode is Apple’s powerful integrated development environment for creating great apps for Mac, iPhone, and iPad. Xcode 4 includes the Xcode IDE, instruments, iOS Simulator, and the latest Mac OS X and iOS SDKs.
Electron - Build cross platform desktop apps with web technologies
Kony - Kony is a mobile application application development platform that facilitates companies to deliver innovative mobile solutions.
GTK - GTK+ is a multi-platform toolkit for creating graphical user interfaces.