A/UX - Apple UNIX
A/UX was Apple's solution to Sun workstations in an attempt to break into the business and server market. Since the Macintosh could only boot into its standard system software, the UNIX layer ran on top of System 7 as seen in the screenshot image (A/UX 3.1.1, the latest version created).
A/UX 3.1.1 required a 68030 CPU with FPU. A popular choice was the Mac IIci with cache card, or a Quadra 950 which offered the most powerful solution. A/UX does not run on all 68040 based systems. In particular, AV systems such as the 840av will crash upon boot. 68LC040 based systems are incompatible due to the lack of FPU, and interestingly enough when a full 68040 CPU is installed in some of these systems it was still unable to run.
I have personally run A/UX 3.1 on a IIci and Quadra 610. Failed attempts include the 840av and LC475 with full 68040. My next attempt will be an SE/30 which are known to run it just fine, albeit with a memory upgrade.