rgd1101 :
because there isn't any "legit" emulator.
Emulators are perfectly legal, per the BLEEM case. As is making your own personal copies of the system BIOS and ROM/Disk images under the Fair Use clause.
Downloading ROMS you do not already own is blatantly illegal.
Downloading ROMS for games you currently own is a legal grey area; seriously, pay the money to get the backup kits and disk image software.
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Now that the legal mumbo-jumbo's out of the way:
For each system, these are the best emulator choices:
NES: Nestopia, FCEUGX
Genesis: Fusion, Gens/GS
SNES: BSNES*, SNES9x (*BSNES merged into the Higan project)
N64: Project 64 2.1, 1964, Mupen64Plus
PSX: PCSX Reloaded, EPSXE 1.9
PS2: PCSX2
Gamecube: Dolphin
Wii: Dolphin
GB/GBC/GBA: Visualboy Advance-M
NDS: DeSmuMe, Ideas, No$GBA
(*Use latest WIP builds unless otherwise noted)
The best website for up-to-date emulator release builds is romhacking.net.
And anyone attempting to post BIOS/ROM files/sites will be reported. So don't try it guys. [Someone always does].