Lots of countries not labeled "potential nuclear power" which could be at the drop of a hat, if they wanted to... most notably Japan. But really, what could stop, say, Norway from going nuclear?
Not much really, but they would have to violate the NPT treatey; helps to have an already established nuke power industry, though.
Taiwan, South Korea, etc.....these countries could have nukes at a drop of the hat.
Self-control?