by default, mutt has PGP (and, therefore, GPG) support compiled in, and gpg directives can be added to your .muttrc file
see section "XIV. Configuring Mutt For Use With GnuPG" of the above for more info
pine:
There are a few different packages that aid pine users with encryption
Pine comes with pinegpg filters for encrypting and decrypting, signing and verifying
Pine Privacy Guard http://quantumlab.net/pine_privacy_guard/ is a perl script that works as input and output filters to pine and is useful on older versions of pine