FLTK (pronounced "fulltick") is a LGPL'd C++ graphical user interface
toolkit for X (UNIX), OpenGL, and Win32.  FLTK was designed to be small and
modular enough to be statically linked, but also works fine as a shared
library.  FLTK also includes FLUID, an interactive user interface builder
program.

FLTK2 is the next generation of FLTK and still in very active development.