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

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/

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

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

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/

Emu28
An open source HP18C/28C emulator for Windows.
http://privat.swol.de/ChristophGiesselink/emu28.htm

Emula3
Contains emulators for many different computers and calculators/PDAs for many different platforms. [English/Italian]
http://www.emula3.com