MailTags is integrated with Spotlight, and its functions can be incorporated into rules and smart mailboxes. Create a smart folder based on a project, a due date or a priority. I'm in the midst of transferring a domain I own, and I easily added a project tag to all the related email messages. A simple Spotlight search turned them all up for me in an instant.
There's more, of course. I've only been using it for a short while this afternoon, but I can see how it could be pretty useful. MailTags requires Mail 2.0 and Tiger. A $20US donation is suggested.
[Via I Heart Apple]