IBM had their own P/390 systems that basically put a mainframe on a PCI card to go in a PC or RS/6000 host. There were also the FLEX-ES systems that were Linux

IBM had their own P/390 systems that basically put a mainframe on a PCI card to go in a PC or RS/6000 host. There were also the FLEX-ES systems that were Linux-based mainframe emulators running on Intel hardware. IBM sanctioned and allowed this for long enough for a number of companies (including mine) to buy them, then pulled the plug on the program and forced everyone to buy real mainframe hardware instead.