My apologies for contradicting you, but this is too simplistic an answer. The answer depends on the laws in your state, whether it was coded on your own time, whether it was coded with any resources of their company, whether ideas in your product can be shown to possibly have come from the work you did for the company, and what your NDA/employment letter/employee agreement states. Unless you run that document by a lawyer and get specific set asides carved out, they might very well be able to take you to court. Some states are really good for you though, like CA. FWIW, I am not a lawyer, and this does not constitute legal advice.