VGB - Gameboy Emu - works great.
VGBA - Gameboy Advance Emu - sucks hard for most games even when overclocked to 1.15Ghz from the default 600Mhz.
Picodrive - Genesis Emu - Awesome Possum, works very well.
DrNokSnes - Super Nintendo Emu - Works very well aside from laggy controls in games like Super Metroid.
iNes - Nintendo Entertainment System Emu - Works very well
SDLMame (I think its called) - Multiple Arcade Machine Emulator - Lacks GPU Acceleration so it doesn't play most games to well.
The Colecovision emulator works very well also, forgot the name though.

There's probably more, but those are the ones that come to my head.

Also, try to use the search feature so that nobody blasts you for not using the search feature instead of just taking 37 seconds to help you out.