PostScript
The YAP for PostScript previewing is used to dynamically edit and re-render PostScript as if one was editing a file, unlike the actual PostScript interpreter itself (such as ghostscript), which deals with PostScript input interactively from a user.
YAP first was bundled with the NeXT demos package of NeXTSTEP, which was also bundled with OPENSTEP. It was not carried into its incarnation as Mac OS X, presumably because of PostScript's lessened importance with the OS X subsystems relying on Display PDF instead.
Read more about this topic: Yet Another Previewer