Organizing your e-mail can be difficult. There are Smart Mailboxes, of course, but what if you would like more arbitrary control?

Apple’s Mail app includes seven flags of indifferent colors (a bit like the old Finder labels, ahem). But what if you can’t remember what each color represents? Once you have flagged a message with a given flag, you will see a mailbox for it appear under Flagged. Click on the triangle next to Flagged to see the mailbox associated with each Flag.

Each of the mailboxes may then be renamed by right-clicking on it. The new name for the flag will now appear everywhere that the flag’s name appears.

[crarko adds: I assume this is Mavericks only, but I could be forgetful.]

Source: Mac OSX Hints