Ambrosia Software (http://www.ambrosiasw.com) has released EasyEnvelopes for Mac OS X. It’s an US$9.99 solution for envelope printing needs.
It’s designed to facilitate tasks whether you’re printing a one-off envelope, or a stack of holiday greeting cards. You can find it at http://www.ambrosiasw.com/utilities/easyenvelopes-mac/ .
With EasyEnvelopes you can pick from existing standard envelope sizes, or design your own from scratch. You have complete control over the formatting of your envelopes, from the location of the addresses right down to line by line font/size/style control. You can even print the return address on the back of the envelope, or include USPS barcodes for quick delivery.