Calc98

Calc98 is a small but very powerful scientific calculator, with lots of features:

Version 5.3 (totally free)

RPN

Base-n, hex, octal and binary modes

Scientific/Engineering Functions

Matrix/Array Functions

Complex Numbers

Statistical Functions

Financial Functions

Time functions (calculations, watch, countdown, etc)

Angles (degrees, minutes, seconds)

Roman numerals (!)

Periodic Table of Elements (!!)

Units Conversions

Physical Properties and Constants

Editing Properties, Units and Constants

System Functions

Miscellaneous Functions

Keyboard Accelerator

International Versions

Version 5.6 is cheap and has a lot of new functions:

Better RPN logic and visible stack

New modes for fractions and feet/inches

IP address calculation mode

Better financial functions

New interactive financial rates dialog

Customised calculator themes

More data than ever, including new particle physics data

Updated matrix and complex modes

etc.