VMS 5.5x

The Virtual Memory System (VMS) is an OS originally designed for the DEC VAX line of servers. Over time, the OS grew to support Alpha and Itanium based machines. Released initially in 1977, the OS was a major competitor at the time to UNIX based machines.

Dave Cutler and other VMS designers were later hired by Microsoft to work on the NT kernel after an internal DEC project didn't go anywhere. Many design decisions in NT were heavily influenced by VMS. DEC, not liking MS was poaching its employees, was given a deal by Microsoft giving DEC compensation in the form of collaboration.

(Insert stuff on how cool VAXes are)

The name later changed to OpenVMS, which is actually not really open source but uses a shared source model, that is, the source code is available, at a price. An x86 port is in progress. The most recent version is OpenVMS 8.4, which is still supported by HP.

You may need a license key to actually boot VMS after installation is completed. An official hobbyist program provides resources for using VMS. These downloads have not yet been tested. For VAX emulation, try SIMH (for Linux).