FLTK (pronounced "fulltick") is a cross-platform C++ GUI toolkit for UNIX/Linux (X11), Microsoft Windows, and MacOS X. FLTK provides modern GUI functionality without the bloat and supports 3D graphics via OpenGL and its built-in GLUT emulation.

FLTK is designed to be small and modular enough to be statically linked, but works fine as a shared library. FLTK also includes an excellent UI builder called FLUID that can be used to create applications in minutes.