The folks at fournova Software GmbH have published An Illustrated History of Mac OS X. This a wonderful and whimsical collection of images that incapsulates each version of OS X from Mac OS X 10.0 Cheetah to OS X 10.10 Yosemite. Each one incorporates a cat (or other element, in the case of Mavericks and Yosemite) to represent the OS, but each one also includes other reference points that represent the major changes in that version of the OS. Looking at them was an excellent reminder of when things happened, like Safari in Panther and Intel in Tiger. The screen grab below is for Snow Leopard. There's a written description accompanying each image to fill in any gaps. For those keeping score at home, fournova makes Tower, a git client for version control in OS X.
Check It Out: An Illustrated History of (Mac) OS X