To encrypt a message using PGP, you need the PGP encryption package, which is available for free from a number of sources. The official repository is at the Massachusetts Institute of Technology.

PGP is such an effective encryption tool that the U.S. government actually brought a lawsuit against Zimmerman for putting it in the public domain and hence making it available to enemies of the U.S. After a public outcry, the U.S. lawsuit was dropped, but it is still illegal to use PGP in many other countries.

MIT distribution site for PGP
This is MIT's distribution site for Pretty Good Privacy (PGP). You can obtain the latest copy of PGP here and also read about integrating it with mail programs. In addition, there are links to other keyservers, books, and related MIT links.

PGP FAQs
A list of different FAQs regarding PGP.

PGP introduction
This site explains how PGP software works through the use of a public and private key pairs.

Yahoo!'s Pretty Good Privacy page
Yahoo!'s directory of Pretty Good Privacy.