GnuPG

From Grokdoc

Gnupg is a replacement for pgp.

GnuPG is a complete and free replacement for PGP. Because it does not use the patented IDEA algorithm, it can be used without any restrictions.

This program, when use properly, can be a very powerful encryption tool. It interoperates well with email clients such as Evolution, and allows you to digitally sign and encrypt messages and files for privacy.

Once you have set up your keys you may want to smooth the way by installing a separate "front-end" to GnuPG to make it prettier and more user friendly. KGpq is one front-end that this user found useful.

GPA is said to be the more-or-less official frontend to gnupg and is also quite useful.