I kind of miss add-in processor cards to allow a computer to run software from another architecture. Those devices were basically a whole computer on a card or add-on that would use the host computer for I/O with some level of integration. Stuff like the Apple IIe card for the Macintosh LC, or the MacCharlie sidecar for the original Macintosh that allowed it to run PC programs.

I bet there are modern PCI cards out there that can add a hard-to-emulate processor to a consumer/prosumer-grade system, but I'm not sure what to search for.