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

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/

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

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

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/

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

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

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

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

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/