Emulation is the process of simulating the hardware of one computer (particularly the processor) via software on a different one. In other words, it allows you to use software meant for another system that you may not have access to by running a program on your system. For instance, an emulator will let you use an arcade machine that no longer exist or use a game console that can no longer be purchased. Note that this category is different from Games: Video Games: Emulation because the focus here is on the emulation of personal computer systems and Operating Systems running on these.
http://sourceforge.net/projects/agatemulator/
Open source Apple 2 and Agat 7/9 emulator.
http://www.altair32.com/
An open source Altair emulator for Windows.
http://sourceforge.net/projects/cinc/
An open source Bell Labs cardiac emulator for Linux and Windows/Cygwin.
http://homepage3.nifty.com/bugbug/cpm/
An open source CP/M-80 ver2.2 emulator for MacOS.
http://members.iinet.net.au/~tom-hunter/
A CDC (Control Data Corporation) Cyber 6x00, 7x or 17x type mainframe emulator for Windows or Unix.
http://hp.giesselink.com/emu28.htm
An open source HP18C/28C emulator for Windows.
http://hp.giesselink.com/emu48.htm
An open source HP38/39/40/48/49 emulator for Windows.
http://emu51.sourceforge.net/
An open source 8051 emulator for Linux and Windows.
http://hp.giesselink.com/emu71.htm
Open Source HP71B Emulator for Windows.
http://www.gstsoftware.co.nz/
Freeware Arcadia 2001 and VZ200/VZ300 emulators for Windows.
http://www.emuviews.com/
Great emulation page maintained by JoseQ, maker of the famous Rumor Mill, many downloads, great layout - some hosted sites.
http://thomasokken.com/free42/
Open source HP-42S emulator for Android, Linux/i386, Mac OS, PalmOS, Pocket PC, Windows, and Zaurus X/Qt.
http://www.lemoncube.com/26.html
A freeware Creativision, Funvision, and Whizzard emulator for Linux/SDL.
http://gxemul.sourceforge.net/
An open source full-system computer architecture emulation framework for Unix-like systems. Screenshots, download, and documentation.
http://thgsoft.ch/HP15c_EN.html
Shareware HP-15C emulator for Windows.
http://www.jcec.co.uk/
A site dedicated to the emulation of Japanese Machines including X68000, PC6001, PC88, PC98, MSX 1/2/R, Sharp MZ, Sharp X1, Fujitsu FM7, and Fujitsu FM Towns.
http://kcemu.sourceforge.net/
An open source emulator of the KC85 homecomputer series for Linux and Windows.
http://www.ontko.com/mic1/
A Java-based simulator which implements the Mic-1 microarchitecture described in Chapter 4 of Andrew S. Tanenbaum, Structured Computer Organization, Fourth Edition. [Open source, GPL]
http://www.hampa.ch/pce/
Open source Atari ST, IBM PC 5150, Macintosh, and RC759 Piccoline emulators for Unix and Windows.
http://pearpc.sourceforge.net/
An open source PowerPC emulator for Unix and Windows.
http://www.seasip.info/Unix/QDAE/
Open source Apricot emulator for Linux and Windows.
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.
http://www.sol20.org/solace.html
An open source Sol-20 emulator for Windows.
http://sourceforge.net/projects/st20emu/
A freeware ST20 emulator for DOS.
https://github.com/nzeemin/ukncbtl
An open source UKNC (MS 0511) emulator for Linux, Mac OS, and Windows.
http://vmips.sourceforge.net/
An open source MIPS R3000 emulator for Unix.
http://gaia.atilia.eu/
Freeware Videoton TV Computer emulator for Windows.
http://www.6809.org.uk/xroar/
Open source Dragon 32/64 and Tandy CoCo emulator for GP32, Linux, MacOS/PPC, Nintendo DS, and Windows.
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