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.