If you don't know about Thunderbird, then you've been living under a rock. Its a small, lightweight, Email application provided free by the Mozilla foundation. I was looking around for Thunderbird add-ons this evening and found a couple that I found to be a "Must Have" for anyone that uses Thunderbird as their primary email client. I'll list them off in no particular order:

Enigmail - OpenPGP message encryption and authentication for Thunderbird and Seamonkey.

Thunderbird is a pretty great open sourced mail client that is available on multiple platforms and for free. From the beginning its development has been mostly be shadowed by FireFox. The two core developers of Thunderbird have left Mozilla, which is a big blow to the ongoing development of Thunderbird. Will Thunderbird not longer get as many updates or new features? Will development complete stop? Only time will tell.