Cxbx : The World’s First Ever Real Microsoft Xbox Emulator
Cxbx is currently considered as the most powerful Microsoft Xbox emulator which has the more complete features! This may not actually be an emulator to some people but everyone considers it one.
Dolphin : The First Real Playable Nintendo GameCube /Wii /Triforce Emulator for PC /Mac /Linux in the World
Dolphin is an open-source Nintendo GameCube, Wii, and Triforce (the arcade machine based on the Gamecube) emulator for Microsoft Windows, Linux, and Mac OS X (Intel-based).
PCSX2 (PS2 Emulator) : The Best PlayStation 2 Emulator for Windows and Linux
PCSX2 is a PlayStation 2 emulator for Windows and Linux, started by the same team that brought you PCSX (a Sony PlayStation 1 emulator). Currently,with a high end PC it can emulate hundreds of games at full speed !
DeSmuME (YopYop DS) : Working Nintendo DS Emulator that Supports Official, Homebrew and Wireless Multiboot NDS Roms
DeSmuME (formerly known as YopYop DS) is an open source (written in C++) freeware Nintendo DS emulator for Linux, Mac OS and Windows. Its name is derived from emu which is short for emulator, DS and ME.
ePSXe (PSX, PS1 Emulator) : Considered to be the Best Sony PlayStation Emulator
ePSXe is a new freeware Sony Playstation(PSX) emulator that takes advantage of the PSEmu Pro plugin system. It is one of the best freeware Sony Playstation emulator right now.
No$GBA : The First Popular GBA/NDS Emulator
NO$GBA (Nocash Game Boy Advance) pronounced as No Cash GBA is a Nintendo DS / DS Lite and GameBoy Advance emulator for MS-DOS, Windows XP, VISTA and 7.