Thunderbird - Reclaim Your Inbox

Thunderbird 0.8 is Mozilla's next generation e-mail client. Thunderbird makes emailing safer, faster, and easier than ever before with the industry's best implementations of features such as intelligent spam filters, a built-in spell checker, extension support, and much more.

Update: The latest version of Thunderbird is now 0.9. Above link (post title) goes to information about the latest version.