Actually CUPS (the Common Unix Printing System) was an open source project that Apple bought. Contrary to what is listed at CUPS.org, CUPS existed long before Apple bought it.
I understand that CUPS remains Open Source and that (some?) code contributed by Apple works it's way back into the Open Source version available to and used by other O/S'
Note that CUPS is an excellent (though somewhat complicated) web interface to the printing subsystem on the Mac. In some rare cases it is possible to add and manage printers that do not offer a printer driver for the current version of Mac O/S but do offer one that works with CUPS.