Addressix is an envelope printing utility. It's designed to make printing mailing envelopes quick and easy. Even unusual envelope sizes like greeting cards.
It remembers your print settings, so you don't have to choose Manual Feed each time.
It remembers every address you've used, so you can pick common ones (like your monthly mortgage payment) with a few keystrokes. It also lets you use addresses from Mac OS X's Address Book.
It prints bar codes based on 5 or 9 digit ZIP codes (normally the Post Office has to scan the address to do this -- reducing this step can improve accuracy).
And it can print envelopes for everyone in one of your Address Book groups.