ARCHIVED: What versions of Unix can I run on my Mac?

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Depending on the model of your computer and the kind of solution you are looking for, you have several options for running Unix on your Mac:

Mac OS X and Mac OS X Server: Apple's Mac OS X and Mac OS X Server are Unix-based client and server operating systems, respectively. They can run their own applications, as well as many Unix programs.
Darwin: Darwin forms the foundation of Mac OS X. It doesn't have Mac OS X's Aqua interface and won't run Mac OS applications, but it will run many Unix programs, and a graphical interface in the form of the X Window System is available for it. Darwin is open source and is available free of charge for anyone who signs Apple's license agreement. It requires a PowerPC G3 or better.
BSD: For those who prefer BSD Unix but can't or don't want to run OS X or Darwin, FreeBSD, NetBSD and OpenBSD are available. They are freeware projects and offer varying levels of support for 68k, PowerPC, and Intel-based Macs.
Linux: Several Linux distributions are compatible with Apple computers; see ARCHIVED: What versions of Linux can I run on my Mac?
A/UX: A/UX was Apple's first Unix implementation. It is a System V Unix that will run on certain models equipped with 68030 and 68040 processors. It is a true Unix, but can also run most 68k Mac OS applications. A/UX is no longer being developed and is not compatible with PowerPC or Intel-based computers.