CUPS provides a portable printing layer for UNIX-based operating systems. It is developed and maintained by Easy Software Products to promote a standard printing solution and is the standard printing system in MacOS X and most Linux distributions.
You can use EasyEnvelopes to quickly print a one-off envelope, or utilize its tight integration with Address Book which allows you to quickly find a person's address to print an envelope for. It handles USPS bar codes for both 5 and 9 digit bar codes, and boasts many time-saving nicities.
(commercial) Endicia prints mailing labels, envelopes and InstaPostage labels. It integrates with Apple's Address Book, calculates domestic and international rates, has interactive shipping log, and built-in support for electronic postal scales.
a package of printer drivers for Ghostscript and CUPS. This project also maintains the Print plug-in for the Gimp from the same code base
MindCad Tiler scales and prints PDF files on multiple pages to create banners and posters. It automatically scales and centers PDF image as you add or remove rows and columns.
pearLabelizer is a little utility that makes it a breeze to print addresses or other text onto specific labels of a label sheet.
An OS X Cocoa service that allows you to 'print selection' in Cocoa applications.
PrinterSetup is a flexible printer setup system. It is suited to environments with more than 20 deployed printers. It is designed to work with CUPS on Mac OS X. PrinterSetup is a component of PrintingWorks, a print accounting solution.