A spent a few days designing and programming a calculator app that I thought would be intuitive and simple. It utilizes a single gesture control as well as shake detection. It's free for iPhone: http://itunes.apple.com/us/app/numbrs/id517346105?mt=8<p>If you download it, let me know what you think.
Really cool!<p>Are you thinking of adding new features because you could add sine (and sine-1), cosine(cos -1), square root, tangent (tan -1). This could have a lot of potential.
This may be the wrong time/place to mention it, but I was looking for an online calculator some weeks ago and could not find something that met my needs. I wanted a calculator that would fill in the missing field so I could vary time, interest and principal. For example: How much per month do I need to save at 5% interest to have a million dollars in thirty years? How much if you make more interest? How much to do the same thing in twenty years?<p>Anyway, best of luck/just tossing out ideas.