X Windows

X11 - the number 11 refers to the current Version of the X Protocol - is one of the most sophisticated windowing systems. Although it is a platform independend system it is the windowing system of choice in most Unix flavoured systems.
Writing a GUI is very often done in Java in these days - for good reasons - but there is still a lot of X11 GUI developement. In my personal opinion, Motif is still much more elegant.

Reasons for using X11 as UI programming platform

• X11 is not only a UI. It is actually a network protocol. Network based displays are system inherent to the system and need no special tools for display. In X11 the so called X - Server is handling that.
• X11 is part of the C/C++ world. It uses the same terminology and follows the same leading ideas.

Literature

X Lib Reference is a good Reference on X11 lower level programming. If you want/need to dig very deep into the X11 library, this a must have. If you want to use a more powerful framework, you will not need it.
OSF/MOTIF Programmer's Guide along with OSF/MOTIF Programmer's Reference explain the concepts of widgets and other building blocks of X11/Motif.