wxWindows is a cross-platform C++ GUI development library. Besides classes for windows, frames, dialogs, controls, etc., there are classes for sockets, threads, a couple data structures, date/time, files, file systems, and many others.
It runs under win32, Motif-based X-Windows, and GTK+ -based X-Windows. It also runs atop the MGL portable graphics library on a wide variety of platforms, including QNX.
A minimal wxWindows application