FLTK (pronounced "fulltick") is a cross-platform C++ GUI toolkit for UNIX/Linux (X11), Microsoft Windows, and Mac OS X. It 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, and also works fine as a shared library. It also includes an excellent UI builder called FLUID that can be used to create applications in minutes.
Prima is an extensible Perl toolkit for multi-platform GUI development. Supported platforms include Linux, Windows NT/9x/2K, OS/2, and UNIX/X11 workstations (FreeBSD, IRIX, SunOS, Solaris, and others). The toolkit contains a rich set of standard widgets and has emphasis on 2D image processing tasks. A Perl program using Prima looks and behaves identically on X, Win32, and OS/2 PM.