The best apps/sites are the ones that do all the work for you. I am not a big fan of mathematical computations. So I favorite every calculator out there (LED, resistor, wattage, amperage, voltage, metric,) anything that quickly easily answers a common question. You would also provide the formulas for those who would like to learn how it works. For me this is not about laziness. I have worked on many Arduino projects and usually these calculators are used to help move the project along.
Select as Best AnswerUndo Best Answer
Why, are you trying to develop something that will sell? I would say think about what hasn't already been done. But i think the best thing is to look at what is already out there and see what you can do to improve upon it. What can you do differently that might get people interested in picking up your offering.