One of the best things about computers is that they never really "die". If original parts become unavailable, newer ones can frequently be substituted; even if entire systems are lost, as long as documentation and logic diagrams survive, they can be recreated from scratch (inaccurately, yes, from an absolute historical point of view, but operational nonetheless) using modern technology.
Another technique that can preserve the legacy of antique systems is simulation. Even better, it's a technology that's available to everyone who owns a modern computer (even a PC)! In this environment, the entire historical computer system is simulated in software running on another computer. The two computers don't even have to be remotely related to one another.
Through the efforts of Mr. Robert Supnik, simulators are available (in C source code, no less) for various DEC processors and the DG Nova. The machines supported are: DEC's pdp1, pdp4, pdp7, pdp8, pdp9, pdp11, pdp15, and DG's NOVA.
The package, complete with operating systems, is available from: ftp://ftp.digital.com/pub/digital/sim/ .
As with many things, if you're not on one of the platforms that the simulators were designed for, you'll need to port it over. Mr. Supnik states that the simulators work under VMS and OSF1; I can confidently affirm that they run under Linux, CLIX, and AIX as well. My hat's off to Mr. Supnik for graciously providing these wonderful programs!
Judging from what I've been hearing on computer- related USENET groups like "alt.folklore.computers " and "alt.sys.pdp10 ", simulation of antique architectures is a bit of a sore point with a lot of purists. If you take umbrage with any of my opinions here, feel free to flame me at my address below.