E-mal cient Thunderbird 3 is now out but I can't rate it as fabulous. I have used Eudora as my main e-mail client for many years. It's no longer being supported and many recommend a move to Thunderbird.
Thunderbird 3 is now out and the Mozilla folks have some cool features. One is for the situation where you promise to attach a file to a message and then forget to attach. Thunderbird 3 will scan your message for words like 'attachment' and then prompt you if you don't actually attach anything.
The tabbed model, a la their web browser Firefox is also cool. But they still don't have a separate line for CC or BCC when you create an e-mail. Any addresses go to the To field by default and then you have to move to CC or BCC. That's too kludgy.
I wonder about the future of e-mail clients when so many seem to be going to the cloud (gmail, etc) for their mail.