Many emulators and simulators have been written for old computers. Emulators are generally able to run the software from those computers, while simulators may simply simulate the operation of that particular computer. Below is a collection of emulators and simulators, some of which may no longer run as-is.
The Hercules System/370, ESA/390, and z/Architecture Emulator
ENIAC Simulator from Drexel University, ENIAC Simulator from Mike Riley
Colossus Computer Simulator (Virtual Colossus)
Manchester Small Scale Experimental Machine “Baby” Simulator by David Sharp, University of Cambridge Java-based Simulator of “Baby”
Simulators for various U.K. computers (EDSAC, Ferranti, Elliott Bros., etc.)
Moonjs: An Online Apollo Guidance Computer (AGC) Simulator, Virtual AGC and other Simulators