Older Unix GUI toolkit, and the best looking one (according to some developers) developed on top of the Xt intrinsics. A real PITA to program for, but that's Xt (and, for that matter, C) for you.
IMO, if you've got a choice develop for something else (Tk, GTK, Qt, anything!) rather than Motif or the other Xt-based toolkits (the most widely known one of which is the Athena Widgets, whose main use these days seems to be xterm...)