I've used emulators for these:
Sega: Master System, MegaDrive/Genesis+MegaCD/Sega CD+32X, Game Gear (and rarely SG 1000 and SC 3000)
Nintendo: NES(/FDS), SNES, N64, GB(C), GBA, DS (and rarely Virtual Boy)
Sony: PS1, PSP
Atari: 2600, 7800, Lynx
Arcade: CPS1 & CPS2
Other: Commodore 64, Apple II, Mac, Odyssey2, ColecoVision, NeoGeo (CD), WonderSwan (Color)
I may have missed a couple. Oh, and obviously DosBox.
Recently I've been playing a fan translation hack of Dragon Quest VI on SNes.