GnuPG (GNU Privacy Guard), is a Free alternative to PGP (Pretty Good Privacy), a tool that uses public and secret key cryptography to protect data.
GnuPG can be used to encrypt, decrypt, sign and check files using several algorithms.
GnuPG's authentication is based on the web of trust, an informal network of trust relationships between independent users.
There are several graphical tools for GnuPG: Kgpg, GNU Privacy Assistant, Seahorse.