Regardless which platform you are using, theres bound to be a calculator included in the stock OS. For simple calculation, these calculators work fine, but if you want to do more complex calculation, like solving an algebraic expression, it wont make the mark.
I just posted my first application to the Google Play Market.
It is called QOS Tip Calculator. It is a tip calculator that allows you to factor in the quality of service you received from your server to determine the tip. Its a novel idea in my opinion, and for all I know it is a fairly unique idea...
I am making a calculator lockscreen widget for android, but the screen timeout is not reset during interactions with my widget so even while you are in the middle of using the calculator, the screen will timeout and turn off. This is pretty annoying.