I'm using Thunderbird 3 now and I like the new features. Searching your e-mails is faster and your results shows up in Tabs, like in a Tabbed Web Browser. Those are just my favorites though...
What's New In Thunderbird 3?
Tabs and Search
If you like Firefox's tabbed browsing, you're going to love tabbed email. Thunderbird 3's tabbed email lets you load emails in separate tabs so you can quickly jump between them.
Search results open in a new tab too. New tools like our timeline and filtering tools will help you pinpoint the email you're looking for, whether it's the one from yesterday, last month, or several years ago.