If you use Apple's Mail application on your Mac, then you have to check out MailActOn.
It is a free extension that allows you to manage and control your e-mail in spectacular ways:
http://www.indev.ca/MailActOn.html
And here are some really smart tips on how to use MailActOn from our the savvy folks at BlueFlavor.com:
"Get Control of Your Inbox"
http://www.blueflavor.com/ed/tips_tricks/get_control_of_your_inbox.php
