Server OSes are the Formula 1 of operating systems. Key technology is initially developed for server environments that will eventually trickle down into the mainstream. For example, multitasking, multithreading, and usage of the Internet Protocol was first available in mainframe systems long before consumers saw them in their desktop computers. A flourishing and innovative developer community has spawned hundreds of different operating systems and flavors, more than 400 for Linux and 10 different trees for UNIX alone. We travel back in time and revisit the most important milestones of the server operating system.