Nesoid for NES/Famicom, Snesoid for Super Nintendo, and I think it's called Gensoid for Sega Genesis, though I don't use that one. All available on the Market as 'lite' and full versions. Haven't come across any others yet, all three of these are rolled up by the same developer and seem to work pretty well.
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be forewarned - they all have a 'virtual dpad' for touch-screen-only models, but it's a *pain* to use, YMMV.