Hercules
Hercules is an open source software implementation of the mainframe System/370 and ESA/390 architectures, in addition to the latest 64-bit z/Architecture. Hercules runs under Linux, Windows, Solaris, FreeBSD, and Mac OS X. There are multiple versions of Hercules available, some of which are maintained more actively than others.
These pages describe additional functionality that I'm developing for Hercules.