Ghostscript

Ghostscript is an implementation of the Adobe PostScript page description language. It includes functions to render PostScript and PDF to printers or to the computer display. A helper program called Ghostview allows scrolling, paging backward, and zooming the display.

Originally developed by Aladdin and released under the "Aladdin Free Public License", the copyright is currently owned by artofcode LLC. Two versions are released one after the other, the first for commercial use with the original AFPL license as AFPL Ghostscript, and another with the GNU license as GNU Ghostscript.

The GNU version is also used as the basis for a Display Ghostscript, which adds the functionality needed to fully support Display PostScript.

Links:

Ghostscript, Ghostview and GSview (http://www.cs.wisc.edu/~ghost/)
Display Ghostscript (http://www.gyve.org/dgs/)