FOX is an Extensible C++ Class Library for Multi-Platform GUI development.
Platforms supported include Linux, Windows NT/9x/2K, and UNIX/X11 workstations
(IRIX, Solaris, HP-UX, Tru64, AIX, FreeBSD, and others).
Conciseness of the Fox toolkit has made it easy for me to learn. Two main features popped out for me: The widgets are easy to adapt and the widgets are easy to implement.