wxWidgets - Cross Platform GUI Library

wxWidgets is a C++ library that lets developers create applications for Windows, OS X, Linux and UNIX on 32-bit and 64-bit architectures as well as several mobile platforms including Windows Mobile, iPhone SDK and embedded GTK+. It has the usual basic controls such as text and bitmap buttons, text entry, scrolling list, combobox, checkbox, and so on. Additional to that Powerful event system, Printing facility etc..