Originally Posted by Mr. Alex
Do we have only two toolkits GNU/Linux programs are based on? Or there are more that are used in GNU/Linux GUI?
Some mainstream ones are ncurses (midnight commander), fltk (dillo), openmotif (xpdf?), fox (rezound)... there are probably many more.
Some other pieces of software that can fit more-or-less the concept of "toolkit" are X (yes, X itself), java, wxwidgets, the enlightenment foundation libs, etc.