This article is from the alt.security.keydist FAQ, by Michael Bauser firstname.lastname@example.org with numerous contributions by others.
GPG is available at http://www.gnupg.org/
PGP is available at http://www.pgp.com/ and http://www.pgpi.org/
Puffer is available from http://www.briggsoft.com/
RIPEM is available at http://www.cpsr.org/cpsr/privacy/crypto/tools/ripem/
InvisiMail and Sifr are apparently no longer available.