Lambda the Ultimate

SIMH, the Computer History Simulation system, is a behavioral simulator for obsolete systems of historic interest. Originally intended as an educational project, it is increasingly being used in long-lived production environments as a substitute for real systems. SIMH is continuously being extended to simulate new machines.

This item isn't directly PL related, but since many LtU regulars are fond of programming language history, I assume there is interest in other apsects of computing history. On top of that, if and when you try to rescue an obsolete language implementation, there's a good chance you are going to need something like SIMH.

This ACM Queue article describes the design issues invloved in building SIMH, and the problems the arise when simulating old hardware systems.