Emulators

Altair32
An open source Altair emulator for Windows.
http://www.altair32.com/

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

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

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

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

Emu28
An open source HP18C/28C emulator for Windows.
http://hp.giesselink.com/emu28.htm

Emu48
An open source HP38/39/40/48/49 emulator for Windows.
http://hp.giesselink.com/emu48.htm

Emu51
An open source 8051 emulator for Linux and Windows.
http://emu51.sourceforge.net/

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

Free42
Open source HP-42S emulator for Linux/i386, Mac OS, PalmOS, Pocket PC, Windows, and Zaurus X/Qt.
http://thomasokken.com/free42/

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

GXemul
An open source full-system computer architecture emulation framework for Unix-like systems. Screenshots, download, and documentation.
http://gxemul.sourceforge.net/

HiRISC Simulator
A freeware emulator for Windows of a system designed specifically for the Applied Systems Programming course at the University of Akron.
http://www.cs.uakron.edu/~margush/307_Old/HiRISC/

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/

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

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/

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

Project8080
Covers the development of a Heathkit H8 emulator on Macintosh.
http://home.comcast.net/~davidwallace2000/h8/project8080_archive/

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/

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

SPRINT
A freeware Sprinter emulator for Linux/i386 and Windows.
http://www.nedopc.org/nedopc/sprinter/sprint.shtml

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

TinyELF
A freeware emulator of CDP1802 based microcomputers for Mac OS X.
http://homepage.mac.com/ruske/tinyelf/

uBee512
Open source Premium Microbee 512Kb FDD emulator for Linux and Windows.
http://sourceforge.net/projects/ubee512/

Virtual Alpha Micro
An open source AM-100 micro computer emulator for Cygwin, Linux, and Mac OSX.
http://www.otterway.com/am100/

vmips
An open source MIPS R3000 emulator for Unix.
http://vmips.sourceforge.net/

XRoar
Open source Dragon 32/64 and Tandy CoCo emulator for GP32, Linux, MacOS/PPC, Nintendo DS, and Windows.
http://www.6809.org.uk/dragon/xroar.shtml