FLTK (the Fast Light Toolkit) is the most recent framework to have become available on Haiku, although it seems we've had Dillo for ages ...
FLTK Text Editor - Simple text editor using the FLTK toolkit.
rundlg_fltk - A standalone FLTK run dialog program
Tada - A simple FLTK GUI todo app written in x86-64 assembly
wcout_fltk_demos - Two FLTK demos