This article is from the Where to get the latest PGP FAQ, by Michael Paul Johnson email@example.com with numerous contributions by others.
Gnu Privacy Guard (GPG) is a serious OpenPGP standard competitor to PGP,
but really it is more of a growth from the initial Gnu Public License
versions of PGP itself, with some independently-written code added where
necessary. It is a serious alternative, and quite secure.
S/MIME is gaining a foothold on the secure email market, but my
experience with it has been rather negative. Current implementations of
S/MIME (1) don't always use secure key lengths, (2) often require
payment of an annual fee to a central key certification authority, (3)
have much more limited key management facilities than PGP, and (4)
usually don't have source code open to inspection like GPG and most
versions of PGP. On the positive side, S/MIME is integrated into email
packages like Microsoft Outlook and Netscape Messenger.