CalcEdit is a simple calculator for the iPad, with a couple powerful features.
Each calculation is added to a rolling history. You can edit any entry in the history and calculate the new result. If other calculations depend on this result, they are updated as well.
Also, you can tap any result line to add its value to the current calculation. Think of this as a touch-friendly type of memory; instead of explicitly saving results to a limited number of memory slots, you can just tap on any previous answer to use it.
This version contains both basic and scientific modes, including common trigonometric and combinatorial functions.