Hercules with Jason UI, emulator of IBM mainframe
Hercules is an open source software implementation of the mainframe System/370 and ESA/390 architectures, in addition to the new 64-bit z/Architecture. Hercules runs under Linux, Windows (98, NT, 2000, and XP), Solaris, FreeBSD, and Mac OS X (10.3 and later).
Jason 1.00 is an integrated graphical frontend to the Hercules S/370, ESA/390 and z/Architecture Emulator. What, you haven't heard of Hercules before? It's a masterpiece that emulates IBM mainframes, from old good IBM System/360 and up to the modern z Series... No, it has nothing to do with IBM compatible... No, it can't emulate Xbox 360... Oh, you are asking what a mainframe is? Then probably you don't need Jason.