Enigmail

An extension for Thunderbird and SeaMonkey, Enigmail is a must-use for anybody who's concerned about sending e-mail that can be read by anybody, including your Internet Service Provider. It's not intended to encrypt all of your mail, although you can certainly do that. Rather, it's best used for ensuring that even remotely sensitive e-mails don't get read by the wrong person.

It uses the OpenPGP standard to digitally sign your e-mail, and can be configured to accommodate multiple e-mail accounts. One hang up is that it's impossible with the current version to permanently decrypt e-mails, which can prevent them from being found by the otherwise-powerful Thunderbird search tools. Enigmail is an invaluable resource for messages containing sensitive information. An installation guide, which is essential for all but the most advanced power users, is available at the Enigmail Web site.