I just installed Thunderbird 3.0, and after a quick look-around, think it has some nice new features. Read about them and download the installer file here: http://www.mozillamessaging.com/en-US/thunderbird/
Those of you using Outlook Express or Windows Mail and considering moving to Windows 7 should be aware there is no email client included. The most popular moves have been to Windows Live Mail and Thunderbird. You'll need to download and install either one (or both), and import your messages and contacts. If that doesn't make much sense to you, contact me and we'll do it together.