This app was mentioned in 1 comment, with an average of 3.00 upvotes
Tile Molestor and YY-Chr are the only tools I've personally used, but those are for direct editing and viewing for a game's encoded graphics. For pixel art in general, there seem to be many apps like this one. However that would just be used for concept art; I don't know of an app that can take what you draw and convert it into the graphics format the NES hardware requires. I'd be interested to see if that exists though.