Ghostscript, Ghostview and GSView

This software is available for almost every OS and lets you view PS and PDF files.

Web-site: http://www.cs.wisc.edu/~ghost/

Adobe Acrobat Reader

PDF browser, available for MS Windows and Linux.

Web-site: http://www.adobe.com/products/acrobat/readermain.html

XPDF

PDF browser, available for MS Windows and Linux.

Web-site: http://www.foolabs.com/xpdf/

Linux

In many Linux distributions it is possible to view PS files under X Window with gv, PDF files can be viewed with xpdf.