SIMH

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 is well emulated by it. It can even emulate network interfaces for the PDP-10, PDP-11 and VAX, so the emulated systems can network directly onto the internet, if the emulated operating system supports it, like BSD or VMS.

Altair Z80

SIMH emulates the Altair architecture with your choice of either a Z80 or Intel 8080 processor. It can run CP/M.

DEC PDP-1

The original PDP-1 is emulated well, and a variety of software for it runs.