Ask HN - Emulators for legacy systems?

So, I was reading through some old papers on legacy systems as well as some documents and code samples from an assortment of older systems either I or a family member played with over the years. And I got to wondering, are there collected resources that cover emulators for legacy systems?

For instance, I know there used to be Baby/400 for AS/400 but that seems to have gone the the bit bucket in the sky. OpenVMS is still around for the VMS fans.

I'd be curious if collections exist out there? My searching so far for specific systems has been hampered by being a bit busy with other projects.