I've been working on a port of my IronBabel framework (universal emulator platform) to JS for a couple months whenever I can spare a few minutes. Designing it properly has been a challenge, but the goal is that any computer can be emulated using it. The focus is on simplicity and handling common tasks, e.g. generic memory mapping, JIT code caching, etc.

From here, what I'd love to do is this: develop emulators for common platforms in the clearest way possible (no optimizations -- that can be separate) and simultaneously document the hell out of older platforms. In the end, you'd theoretically have a repository comprised of a set of documentation and a set of "reference emulators".

It wouldn't be fast, but it'd preserve these platforms effectively forever.