I have a .Mac e-mail address, and I like the aliases feature, but I'm annoyed that they all use the same sender's name (there's no way to specify a different name for each alias). I don't want my personal e-mails to have my business name or vise-versa.
It turns out that there's a relatively easy fix available.
Open your Mail prefs file (quit Mail first), located in ~/Library -> Preferences -> com.apple.mail.plist, with Property List Editor (easiest, though any text editor will work). Navigate into Root: Mail Accounts, and then find the number that corresponds to your .Mac account (check the Account Name in each numbered section to find it -- my .Mac account was '1'). When you open the number for the .Mac account, you will see the EmailAliases key. Open that, and you'll see numbers again, with each one coresponding to a different email alias. You will see a blank entry for name in each alias; just fill that in with what you want for each alias.
Save the file and quit the editor when you're done editing. Launch Mail, and you'll find your aliases now have names associated with them in the Account pop-up menu. I haven't tried it, but I don't see why you couldn't add more aliases this way ... and I also don't see why it wouldn't work with other types of accounts.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20051213200935504