Multics (Multiplexed Information and Computing Service) was an early time-sharing operating system for mainframes and minicomputers which has influenced every operating system created since, especially via Unix (the name of which is a play on words of Multics!).
Multics is dead. There are no systems running it today (the last one was shut down in 2000. Unfortunately there are also no emulators for Multics, however much of the source code is extant, so should a viable emulator be written (a non-trivial task), it is possible that Multics could be emulated.
First, porting Multics would be a Herculean task. The design isn't well-suited for porting. It used specialized hardware built especially for it, this hardware had many features which do not have exact analogues on any other platform today. Even if that could be overcome, Multics was written in PL/I, of course. There are no PL/I compilers available other than commercial ones aimed at large enterprise systems, and which aren't getting any cheaper. The lack of a free Unixland PL/I compiler inhibits any porting task.
There have been a few attempts to implement an emulator for one of the systems Multics ran on (usually a Honeywell 6180.) However, at the time of writing, I know of none which is capable of running Multics in any meaningful way. This is apparently tied to the exotic hardware required to support Multics, which is not very well documented.
There is an emulator of sorts here though it hasn't been updated in over a year, and apparently it's limited to running simple bootloaders for the time being.
-- Lucky 14:00, 20 July 2009 (UTC)