For a system evolved from a BSD Unix the Mac OS does not get much respect from the open source community.
There are Mac-only programs in the open source firmament. AppleJack is a nice troubleshooting assistant. The XBMC media center . has no counterpart in the Windows world. Fink connects the Mac to the Linux open source mainstream.
NOTE: The XBMC folks write to say that there are versions of their software for both Windows and Linux, as well as the Mac.
But most of the popular Mac open source products out there are familiar to Windows users. These include the Mozilla Foundation's Firefox and Thunderbird, Gimp, and the VLC Media Player.
Why hasn't open source made more of an impact in the Mac universe? Following are some theories. Feel free to add your own, or just heckle:
Mac people are users. As opposed to programmers. When your initial bundle is filled with usable software who needs to go into the code?
Steve wouldn't like that. Apple is not friendly to open source. Their attitude makes the relationship between Microsoft and open source look like a first-class bromance.
No critical mass. I don't mean there aren't enough Apple users out there, I mean there aren't enough angry ones. Linux people know they're on their own, and Microsoft frustration abounds. Are you an angry Mac user? Do you have any other friends?
No profit in it. A lot of open source effort is driven by the profit motive. How much money can one make in Apple open source?
Apple gets there first. Apple is quite adept at exploiting new niches within its own ecosystem. The capabilities of its operating system are full available in the commercial market.
Mac people are upscale. Since Macs generally cost less than PCs, ownership of a Mac shows you are not a penny pincher. This changes the make-or-buy equation, tipping it strongly to buy.
You're not looking hard enough. There's really a ton of Mac open source out there, but PC-using reporters are too lazy to go look at it.