Dave Caolo posted about MailTags last August, but in case you're
unfamiliar: MailTags is a fantastic plugin for Mail.app that allows you
to both tag your messages and create iCal todos right from within Mail. MailTags' abilities go far beyond simply adding
a few keywords though; you can also set projects, categories and priorities for messages and use all this new metadata
to create a far more robust organizational system than simply filing messages in a 'todo' folder. Once MailTags is
installed, a whole new set of search and rule criteria (such as due date, project, keyword and more) are baked into
Mail.app for running searches or creating Smart Mailboxes and rules. It's a pretty amazing plugin, and I urge you to
check out Scott Morrison's Indev site for the full details on
everything MailTags is capable of.
While MailTags 1.2.1 issues a host of bugfixes but no serious new features, I found out about it from this Hawk Wings post in which Tin Gaden states that MailTags 1.3 will have a new feature many users have been wishing for: IMAP tag syncing. So if you use Mail.app and MailTags on more than one Mac, you can soon keep your tags, as well as (I'm hoping) projects and priorities all in sync.
With all that said, go take MailTags for a spin. It's donationware, so show Scott some PayPal love for the hard work he's putting into this most excellent of plugins.