Mozilla Thunderbird is the awesome email counterpart to the ever popular Mozilla Firefox browser. Mozilla has once again created a great, free, open source emailing client. It has built in learning capabilities to combat spam, and can handle multiple email accounts. Aside from that, the Mozilla community also creates addons for Thunderbird, just like they do for Firefox. This means you can extend the already great functionality of the Thunderbird email client to fit more of your personal needs.