Hi guys! Here is my own calculator that I did entirely on my iPhone in two days. I'm actually pretty happy with the result. In my opinion, it is a more useful calculator than the one that comes with iOS. The one downside is that it takes more taps to open my calcultor in Pythonista than a normal calculator app, and with an iPhone 4 and iOS 7, more taps means exponentially more time... :D
You can get it here. It should also work on an iPad, but I wouldn't know because I haven't got one.
Let me know what you think!
Features:

Enter full mathematical expressions with parenthesis (unlike the calculator that comes with iOS)

Comparison support: for example,
sin(pi) > cos(pi)
yieldsTrue

Trig functions and their arc variations

Constants:
pi
ande

Exponentiation, factorization and natural logarithms

Press
=
equal without a current expression to repeat the last calculation (specially useful when paired with theAns
button) 
Handles errors with style (try dividing by zero to see what happens!)

Press the result screen at the top to copy the last result to the clipboard