Numerical — A Beautiful Calculator for iPhone
The tagline for the app is “A Calculator Without Equal”, which is not only clever from a marketing perspective, but also true because the app does not have an ‘equals’ button. It simply calculates answers on-the-fly, and lets you use swipe gestures to undo, redo, or archive an answer for later reference.
The app also has a certain charm to it, with helpful animations and pleasant sound effects throughout (you can see it in action here). It will even give useful error messages, like if you try to divide by zero, as seen in the center screenshot above.