On its road to Mac OS X Apple considered - and in some cases, rejected - multiple operating systems.
An interesting technical history of Apple's OS history has been published and is being made available online as a downloadable PDF.
The history has been written by Amit Singh, and is an extended version of a chapter that appears in his new book, Mac OS Internals: A Systems Approach.
Singh writes: "I strongly believe that reading this document will be helpful in understanding the evolution of Mac OS X, and to some extent, of some aspects of modern-day operating systems. An important reality of technology, and of computing in particular, is that many things that we think of as 'new ideas' are not quite new."
The document describes a "super-detailed" history of Apple's operating systems. Contents include details regarding Apple's earliest OS iterations, its inspiration, the limit of power, and much more.