uCalc is the easiest to use scientific calculator (and its free!) -

Large colored buttons (customizable), recallable paper-tape history, trig/log/exponential using radians or degrees

FEATURES

Large buttons - Color coded by category.
Button color changeable to custom value.
Uses number format defined on your iPhone.
Trig functions
– Sin/Cos/Tan
- Inverse Sin/Cos/Tan
- Hyperbolic
Constants – PI (3.14…) and e (2.718…)
Powers/Logarithms- Natural log , e to power, 10 to power, Square root
Parenthesis- Count of left/right on button
Friendly error reporting
Use degrees or Radians
Delete items or entire input line
Last value or any previous formula can be recalled
Recallable history line
Works in all orientations
Number of output digits can be set (3 to 17 places)

BUTTONS

Large – easy to read and press
Color coded by topic-
- Numeric input
- Trig functions
- Editing (clear line, delete last, recall last value)
- Operators (+, -, /, *, ^, parenthesis)
- Other functions

HISTORY LINES

Scrollable history region above input line.
Each line displays input formula + calculated answer.
Any line may be deleted.
If formula is started then selecting a line inserts the value.
Selecting a line with empty formula brings down the entire formula.