iota-calc: The Programmer's Calculator

Key Features:

Supports a rich expression format, which is in most cases identical to what you would type in your programming language.
Support for adding unlimited numbers of variables.
Built-in debugger that lets you see the expression simplified step by step.
Debugger Back-step lets you backup as many steps as you like so that you can replay critical operations.
C/C++ operator precedence
Mouse optional, all calculator commands and functions can be done using the keyboard alone.
Mini-Calc Mode lets your calculator take up very little screen real estate, and yet remain entirely usable.
Numerous built-in unit conversions (memory sizes, lengths, areas, volumes, temperatures etc.)
Built-in great circle route and straight line distance calculators.

Binary, Octal, Decimal, Hexadecimal, Booleans, Characters, UTF-8 (out only), and UTF-16 (out only)
Convert automatically between these types in your expression with the keyboard, a button, or a contextual menu.

Standard: Addition, Subtraction, Multiplication, Division, Modulus, Power, Square Root
Grouping: Parenthesis
Variables: Previous Answer
Bitwise: complement, and, or, xor, shift left, shift right
Comparators: equals, less than, greater than, less than or equal, greater than or equal
Logical: and, or, and not
Logarithms: Natural logarithm, log base 10, log base e, e raised to x
Trigonometric: Sine, Cosine, Tangent, Hyperbolic-Sine, Hyperbolic-Cosine, Hyperbolic-Tangent, Arc-Sine, Arc-Cosine, Arc-Tangent (2 Variants), Degrees To Radians, Radians to Degrees
Other: Min, Max, Round, Ceiling, Floor, Absolute Value, Sign, Random

I use iota-calc pretty much on a daily basis to plan out calculations for programming tasks I have. I also use it when helping my son with his homework. -- Steve Mayer
Very very useful little app, well worth the money the author is asking for! -- ScottM review of Mac OS X version on MacUpdate.com
This is a very nice and unique calc, and they are steadily improving it. I'm giving it a five for current usability, improvements, and a GREAT price. Get it now, before demand drives up the cost. -- MikeyK review of Mac OS 8/9 version on VersionTracker.com