In case you don't know, "Pretty Good Privacy" is "an e-mail encryption program developed by Phil Zimmerman, founder of Pretty Good Privacy, Inc., San Mateo, CA. Published as freeware in 1991 and widely used around the world for encrypting e-mail messages and securing files..." (from here).
Users can download and review the code for free but cannot reuse or modify it. This differs from many open source agreements where users are allowed to modify code and use it in their own applications under certain conditions.
"The PGP 8.0 line includes both Windows and Macintosh versions of the PGP Desktop, PGP Enterprise and PGP Personal as well as a new version of PGP Freeware. The Macintosh products include support for OS X, and the Windows line now supports XP and XP Office."
More here and on the PGP website here.