SIMH consists of simulators for about 20 different machines, and as such, is a very important emulator for anyone interested in computer history; in particular, that of DEC systems, as the PDPs and even the VAX are well emulated by it.
SIMH was primarily written by Bob Supnik of Zork porting fame, and is now widely contributed to by others as well.
The current version is 3.9-0, released on May 3rd, 2012. More up to date developments may be found on their GitHub source code repository.
This is the Altair 8800 emulator which can run both AltairDOS and CP/M 2.2
Emulates the work horse behind the ARPANET IMPs. In 2013 the emulator was modified to bring up the original IMP code and connect them together (tunnelled over IP). Demo instructions are available in the latest source. The IMP side of the 1822 is there, but there is not much of a host side yet. I don't see anything in simh that would be able to plug in anyway. An Associated Computer Consultants (ACC) 1822 card for the pdp-11 would be a start.
I really don't know much about this machine, but I included a way to run the basic1 program I found online. It kind of reminds me of the ROMBASIC back 'in the day'... It's basic with line numbers so I know it's kind of old.. I really don't know much otherwise about this machine.
The Interdata 8/32 micro was the first non PDP-11 to run UNIX. SIMH can run both v6 & v7 research editions for the Interdata.
SIMH also includes Data General Eclipse support, but there is no available OS.
The PDP-11 is emulated enough to run most PDP-11 operating systems including the early Unixes (v1,4,5,6,7 are known to run).
SIMH can run the MicroVAX II with as much as 256mb of emulated ram. Both VMS, NetBSD, OpenBSD and original BSD can run on this emulator. Additionally, the network adapter functions correctly under this emulator, unlike the VAX-11/780.
SIMH can also emulate the first model of the VAX, the VAX-11/780. This is appropriate for older operating systems like VMS release 1.x and the original 32v, 3BSD and all 4BSD releases.
SIMH can either take in configuration parameters interactively, or you can store them in a file. If the file is the name of the emulator+.ini it will load them automatically (eg vax.exe loads vax.ini automatically). The CONTROL+E key will break the emulation and bring you back to the SIMH console where you can alter the running state.
Networking is covered in the Networking with SIMH guide.
While not immediately obvious, there is a mailing list here http://mailman.trailing-edge.com/pipermail/simh/. I would recommend searching the thing, and you'll be amazed how much good information there is in there. Likewise, I'd recommend joining the list if you are interested in running SIMH. You can join here. As 4.0 is still in beta, the public source repository is also a great source of information.