[link|http://www.conmicro.cx/hercules/|Hercules]:

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, Windows NT, Windows 2000, Windows XP, and Mac OS X 10.2 and later.


Lots of links there.

HTH.

Cheers,
Scott.