Today I came across MailTags, a donationware plug-in for Mail 2.0 that makes adding metadata, comments, priority flags and "due dates" to email messages easy. After installing the plug-in, each mail message displays a small "tag" in the upper right hand corner. Click it to reveal the sidebar. From there, you can assign that message to a project you've created, set a due date for action you intend to take, leave yourself a comment in a small text box (like, "Call Julie on this" or "this has been resolved") and more. That's good, but here's where it gets great.
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]