http://members.iinet.net.au/~tom-hunter/
An open source CDC (Control Data Corporation) Cyber 6x00, 7x or 17x type mainframe emulator for Windows or Unix.
http://simh.trailing-edge.com/
An open source collection of emulators for Windows of historical computers, including GRI-909, HP 2116/2100/21MX, IBM 1401/1620/1130/System 3, PDP-1/4/7-11/15/VAX. Also provides software kits, system photographs, and technical papers.