Multi Hardware

Ace
A freeware emulator for Windows dedicated to Capcom system. Sources are also available.
http://ace.emuunlim.com/

CottAGE
Java emulator based on the MAME sourcecode. Download and possibility to play online.
http://cottage.consolemul.com/

E++ - A portable object-oriented emulator
Emulates Space Invaders, Phoenix, Centipede and the home computer Sinclair ZX Spectrum (aka Timex-Sinclair 2000).
http://www.guybrush.demon.co.uk/e/

HiVE : High Velocity Emulator
A Win32 program which emulates classic arcade games such as Galaga and PacMan.
http://hive.speedhost.com/

JEmu
Java based emulator that can play games online.
http://www.gagaplay.com/

Kawaks
A freeware emulator for Windows. Emulates Neo Geo and CPS 1/2 Systems.
http://cps2shock.retrogames.com/

Nebula
A freeware NeoGeo and CPS1/2 emulator for Windows. [English/Spanish]
http://nebula.emulatronia.com/

VAntAGE - Vertical Antiquated Arcade Game Emulator
Written to be as small and fast as possible (to keep the cost of the host PC to a minimum) and to run the arcade games 'as intended'.
http://www.mameworld.net/pc2jamma/vantage.html