MULTICS a.k.a. MIT Project MAC: Multiple-Access Computing
The progenitor of UnixOs.
Multics was never a big success in commercial or installed-base terms, and nowadays it's mostly famous for falling victim to both BigDesignUpFront and the SecondSystemEffect during its inception. But many important OperatingSystem features appeared first or early on it, including I/O redirection (see UnixShellPatterns), the HierarchicalFileSystem, virtual memory, (partial) implementation in a HighLevelLanguage, and AccessControlLists. It went on to host much important software, including MulticsEmacs and the first commercial RelationalDatabase.
June 2008 - the full Multics sources and documentation are now available at http://web.mit.edu/multics-history/ Presently there's no emulator to run them, though.