Visual Calculator is a powerful tool with a full set of arithmetic operations, fractions, brackets, trigonometric functions, logarithms, rounding etc., with functions enabling comments within calculations and variables for intermediate values as well.
 
It is also a fully equipped editor — you have at your command a scrollable and zoomable input area, open/save, undo/redo, select/copy/paste, syntax highlighting, several fonts and colour themes to suit different tastes and a native on-screen keyboard, customized for the quick input of mathematical expressions.
 
There are also a couple of cool special features for improved processing of really massive data. These are autosum tables (which add up their contents) and custom functions (defined by the user).