"wxWidgets gives you a single, easy-to-use API for writing GUI applications on multiple platforms. Link with the appropriate library for your platform (Windows/Unix/Mac, others coming shortly) and compiler (almost any popular C++ compiler), and your application will adopt the look and feel appropriate to that platform. On top of great GUI functionality, wxWidgets gives you: online help, network programming, streams, clipboard and drag and drop, multithreading, image loading and saving in a variety of popular formats, database support, HTML viewing and printing, and much much more"
Very nifty indeed. Works with a varying degree of support with GTK+, Win32, Motif, X11, Nano-X, MGL, MacOS, and OS/2.