Chip-8 Emulator

A freeware Chip-8 emulator for DOS.
http://members.aol.com/autismuk/chip8/

Emula3

Contains emulators for many different computers and calculators/PDAs for many different platforms. [English/Italian]
http://www.emula3.com

Virtutech Simics

A commercial Alpha, PowerPC, SPARC, x86 and x86-64 emulator.
http://www.simics.com/

Evenson Consulting Services' SWTPc 6800/6809 Emulator

Windows 9x/NT GUI emulation of SWTPc 6800 and 6809 systems. A Simulated Machine Environment (SME) emulation plus .dsk file downloads.
http://www.evenson-consulting.com/

Project8080

Covers the development of a Heathkit H8 emulator on Macintosh.
http://members.aol.com/heath8080a/

Solace

An open source Sol-20 emulator for Windows.
http://www.thebattles.net/sol20/solace/solace.html

Altair32

An open source Altair emulator for Windows.
http://highgate.comm.sfu.ca/~rcini/classiccmp/Altair32.htm

vmips

An open source MIPS R3000 emulator for Unix.
http://www.dgate.org/vmips/

st20emu

A freeware ST20 emulator for DOS.
http://sourceforge.net/projects/st20emu/

SPRINT

A freeware Sprinter emulator for Linux/i386 and Windows.
http://shaos.ru/nedopc/sprinter/sprint.htm

cinc

An open source Bell Labs cardiac emulator for Linux and Windows/Cygwin.
http://sourceforge.net/projects/cinc/

Emuviews

Great emulation page maintained by JoseQ, maker of the famous Rumor Mill, many downloads, great layout - some hosted sites.
http://www.emuviews.com/

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.
http://www.jcec.co.uk/

Desktop Cyber

An open source CDC (Control Data Corporation) Cyber 6x00, 7x or 17x type mainframe emulator for Windows or Unix.
http://members.iinet.net.au/~tom-hunter/

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.ontko.com/mic1/

Flexemu

An open source EurocomII/V7 emulator for Linux and Windows.
http://geocities.com/flexemu/

Emu51

An open source 8051 emulator for Linux and Windows.
http://emu51.sourceforge.net/

James the Animal Tamer's Emulators

Freeware APF, Aquarius, Exidy Sorcerer, Interact Family Computer, MC-10, NEC PC-6001A, Panasonic Personal Computer, and TRS-80 emulators for Windows.
http://www.geocities.com/emucompboy/

SIMH

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://simh.trailing-edge.com/

CP/M

An open source CP/M-80 ver2.2 emulator for MacOS.
http://homepage3.nifty.com/bugbug/cpm/

KCemu

An open source emulator of the KC85 homecomputer series for Linux and Windows.
http://kcemu.sourceforge.net/

FunnyMu

A freeware Creativision, Funvision, and Whizzard emulator for Linux/SDL.
http://www.lemoncube.com/26.html

Emu28

An open source HP18C/28C emulator for Windows.
http://privat.swol.de/ChristophGiesselink/emu28.htm

blueMSX

An open source SVI318/328, MSX1, MSX2, MSX2+, and Turbo-R emulator for Windows.
http://www.bluemsx.com/

Virtual Alpha Micro

An open source AM-100 micro computer emulator for Cygwin and Linux.
http://www.otterway.com/am100/

TinyELF

A freeware emulator of CDP1802 based microcomputers for Mac OS X.
http://www.tinyelf.com/

GXemul

An open source MIPS machine emulator for Linux.
http://gavare.se/gxemul/

Newbrain Emulator Pro

A freeware Grundy NewBrain emulator for Windows written in Delphi.
http://www.freewebs.com/newbrain/

The Susie V pages

A freeware BCL Susie V emulator for Linux/i386.
http://www.geocities.com/susievem/SusieV.html

PearPC

An open source PowerPC emulator for Unix and Windows.
http://pearpc.sourceforge.net/