I nominate SpeQ Mathematics, a clean and powerful little program done with my favorite programming language (but not by me). It's free, donations much appreciated.
The files need about 450K, half of which is the User Guide.
SpeQ has the following features.
* Expression calculations with arithmetic, conditional, logical and bitwise operators
* Around 100 built-in mathematical, physical and conditional constants
* Around 60 built-in functions for Analysis, Arithmetic, Complex numbers, Hyperbolic, Integers, Probability, Numeral systems, Statistics, Trigonometry
* Define your own variables and functions for use in calculations
* Plot fully adaptable graphs, trace the plotted functions, save your graphs
* Decimal, binary, hexadecimal and octal numeral systems
* Complex number calculations
* Colored workarea
* Save and load sheets
* Orderly overview of all available functions and constants in the Functionstree
* A list with all defined variables and functions in the Memorylist
* Context-sensitive help and a detailed users guidehttp://www.speqmath.com/