Stamps.com has been offering picture postage for Windows for quite some time now, but Endicia (pretty much the only people doing postage service for the Mac) are now offering it for the Macintosh and the software to create it is definitely cool. Check out the software and info on the PictureItPostage page.

If you're unfamiliar with the idea, you take a photo from your iPhoto library and stick it on a postage stamp and then use that to send your mail. Cute, but expensive.