Ghostscript
Ghostscript is a freeware interpreter of the Adobe PostScript and Portable Document Format (PDF) page description languages. It is highly configuarable and can be used as:

- Raster Image Processor (RIP) for raster printerss, for instance, as an input filter of Line Printer Daemon.
- An RIP engine behind PostScript and PDF viewers.
- File format converter, such as PostScript to PDF converter.
- General purpose programming environment.
Several Graphical User Interfaces (GUIs) have been written for use with Ghostscript which permit a user to view a PostScript or PDF file on screen, scroll, page forward and backward, and zoom the text as well as print single or multiple pages.
- Ghostview runs under Unix/X11
- GSView runs under Windows and OS/2
The GPL version is also used as the basis for a Display Ghostscript, which adds the functionality needed to fully support Display PostScript.