Unix. GUIs were first shown at Xerox PARC (Palo Alto Research Center) in 1973. X-Windows is used on Unix to decouple I/O from processing, in other words what you're working on can be displaying on your computer but executing on another. NeXTstep is Apple's Unix developed on/from/with BSD (Berkley Software Division) Unix. I would assume that suntools were developed on Sun Solaris.
