I have at least five emulators on my computer.
NESten which allows you to play NES ROMS. I don't have many. Super Mario 1,2,3 and Megaman 2 but thats about it.
I also have Fusion which is a Sega Megadrive emulator. Its pretty good and plays almost every ROM I've downloaded for it. Truxton, Sonic, Road Rash, Streets of Rage...I have them all.
I also have MAME which has over 150 titles on it plus FBA which is an emulator solely used for Capcom games. Streetfighter II, Streetfighter II Turbo, Super Street Fighter II, Streetfighter Alpha 3 plus some others are among the titles I have installed.
The final emulator I have is WinUAE which is a Commodore Amiga emulator. I have over 50 titles for that with a few others waiting to be unzipped and pasted into the ROM folder. The only problem with WinUAE is that it requires a lot of tweaking in order for it to work properly but its worth it if you grew up with an Amiga (like me).