wxWidgets

wxWidgets is a C user interface development library that allows you to use the same source code to create applications for different platforms, including Windows, Mac OS X, Linux with GTK and X11, WinCE, and more. The graphic interface generated by wxWidgets uses the controls characteristic of each system, so its appearance is practically identical to a native application. wvWidgets only emulates those controls that don't have support, like MDI forms, tree views, and lists. wxWidgets also supports functions including: printing, debugging, multithread, network programming, OpenGL, ODBC databases, etc. You can use it with different programming languages too, like Java, Perl, Python, and Ruby. If you're looking for examples, Kicad and Audacity are two applications that use libraries supplied by wxWidgets.