The viewer is called GhostView on Unix and VMS systems. GhostView uses an engine called GhostScript to interpret the PostScript code. GhostScript is a product of Aladdin Enterprises, available under a free public license. It is also available from GNU under similar terms. GhostView is available from GNU and elsewhere.
Commercial licenses are available for both.
For more information, or access to GhostScript and GhostView, see the GhostScript Home Page .