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.
Open source Apple 2 and Agat 7/9 emulator.
An open source Altair emulator for Windows.
An open source Bell Labs cardiac emulator for Linux and Windows/Cygwin.
An open source CP/M-80 ver2.2 emulator for MacOS.
A CDC (Control Data Corporation) Cyber 6x00, 7x or 17x type mainframe emulator for Windows or Unix.
An open source HP18C/28C emulator for Windows.
An open source HP38/39/40/48/49 emulator for Windows.
An open source 8051 emulator for Linux and Windows.
Open Source HP71B Emulator for Windows.
Emulator2001 and DSVZ200
Freeware Arcadia 2001 and VZ200/VZ300 emulators for Windows.
Great emulation page maintained by JoseQ, maker of the famous Rumor Mill, many downloads, great layout - some hosted sites.
Open source HP-42S emulator for Android, Linux/i386, Mac OS, PalmOS, Pocket PC, Windows, and Zaurus X/Qt.
A freeware Creativision, Funvision, and Whizzard emulator for Linux/SDL.
An open source full-system computer architecture emulation framework for Unix-like systems. Screenshots, download, and documentation.
Shareware HP-15C emulator for Windows.
Japanese Computer Emulation Centre
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.
An open source emulator of the KC85 homecomputer series for Linux and Windows.
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]
Open source Atari ST, IBM PC 5150, Macintosh, and RC759 Piccoline emulators for Unix and Windows.
An open source PowerPC emulator for Unix and Windows.
Open source Apricot emulator for Linux and Windows.
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.
An open source Sol-20 emulator for Windows.
A freeware ST20 emulator for DOS.
An open source UKNC (MS 0511) emulator for Linux, Mac OS, and Windows.
An open source MIPS R3000 emulator for Unix.
Freeware Videoton TV Computer emulator for Windows.
Open source Dragon 32/64 and Tandy CoCo emulator for GP32, Linux, MacOS/PPC, Nintendo DS, and Windows.