The MailTags pane lets you add metadata to your messages.

In everything from Gmail to iTunes to the OS X Finder itself, folders are out—tagging and searching are in. It’s somewhat surprising then, that Apple Mail still hasn’t incorporated tags, even as OS X becomes increasingly adept at on-the-fly searching. Enter MailTags, a plug-in for Mail that allows you to assign tags to your email and thereby organize your messages. 

MailTags adds an unobtrusive icon—a tag, natch—to the corner of your Mail messages. Click the icon, and a panel slides out where you can add tags to your message. Auto-complete will learn your tags as you use them and suggest as you type. MailTags also gives you options to assign custom Project labels and colors, and you can even add To Dos that will sync up with iCal. Tags can be synched to your IMAP server, for access from different machines. The tagging pane can feel a bit cluttered with all the available options, and adding and removing tags from messages requires quite a bit of clicking (which can be somewhat mitigated by a free Mail plug-in from the developer of MailTags that lets you add custom keyboard shortcuts).

By themselves, tags don’t do anything, but when you add in Mail’s Smart Mailboxes, you can easily slice and dice your messages into infinitely configurable chunks. Finding all your messages tagged “Action” from your coworkers or maybe everything tagged “Top Secret” that’s more than a week old becomes effortless. The real value of tags comes in combining them with each other and with the existing data you already have in your mailbox. And since you can use as many tags as you like on each message, you’ll never have to worry about figuring out exactly where to file a particular item.