PGP Extension for Thunderbird
There's a great extension called Enigmail for the Thunderbird e-mail client that provides integration with the GNU Privacy Guard implementation of PGP. Enigmail makes encryption, decryption, and signing activities incredibly easy. It also provides a great tool for searching and importing public keys into a local truststore. I'm going to start using PGP to secure e-mail communications whenever possible. I've published my PGP public key to the PGP Global Directory, and can be referenced directly using the identifier E80DCE23. Here are some screeshots of Enigmail in action:
