One of the great innovations of the NeXt Computer, it made WhatYouSeeIsWhatYouGet really easy, since drawing to the screen wasn't really any different from drawing to a printer. It seems that it is easy to create something, then apply transforms to that something, or reuse the something, just like how in OpenGl you can create a display list, and use it a couple of times transformed and deformed around the scene. (The OpenGL API was based on the PostScript imaging model. PostScript doesn't use a display list, though.)
In MacOsx (which is derived from NextStep), DisplayPostscript has been replaced by a lightweight bitmap-based window server. The Quartz 2D library implements the PortableDocumentFormat imaging model for application frameworks, but is not used in the window server itself. PDF is essentially PostScript without the programming language.
DisplayPostscript was done by Adobe, not by Next. It was closely related to the technology Sun used in the NetworkExtensibleWindowSystem, their early competitor to the XwindowSystem. Unfortunately, Sun got steamrolled by the same bandwagon that gave us CeePlusPlus. Other contemporaneous users of DisplayPostscript included Xerox and at least some universities. I explored replacing the bitmap image classes of the Smalltalk-80 v2.5 image with DisplayPostscript while doing research on portable application frameworks at BrownUniversity? and in the early days of OnTechnology?. I have a vague recollection of bad blood between Next and Adobe, related to hard feelings about business relationship between Apple and Adobe during Steve's Macintosh days. Sun and Adobe worked quite hard to avoid stepping on each other's intellectual-property toes during that period, and everyone was dodging Apple's look-and-feel harassment. OnTechnology? negotiated license arrangements with Adobe for DisplayPostscript, but those were mooted when MitchKapor redirected OnTechnology? in December of 1988. PeterDeutsch, formerly of ParcPlace, and virtual machine wizard, did a Gnu-flavor called "GhostScript" which was still around the last time I looked. -- TomStambaugh