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.