Kalkulator 2.41 - The Mother of All Calculators, period.

The Mother of all Calculators, a Swiss Army knife for scientists and engineers. Perhaps the most capable scientific / engineering calculator for Windows.
On one hand, this is a calculator: evaluates expressions with more than 100 functions (trig / inverse, log / exp, statisical distributions, Euler, conditional, user-defined. Expressions are entered in the customary algebraic notation, and only then evaluated. After computing the value, an expression can be modified or corrected, and recomputed at will. There ara also unit conversions, physical constants, computer math (binary, octal, hex), and function graphs included.
For more advanced users, Kalkulator provides less trivial operations: numerical integration and differentiation, interpolaton (linear or cubic spline), statistics (mean, s.d., histograms, polynomial regression), column operations on stat data, polynomial roots, linear algebra (vector / matrix operations and systems of linear equations).
Hard-to-find features: systems of non-linear and differential (ODE) equations, multi-argument function extremum search and point or histogram fitting with an arbitrary function.
Most importantly, advanced features don't obstruct the basic ones. You can use as much of Kalkulator's capabilities as you need, without being hampered by the program capabilities you do not need. Disk save / restore, Help file and other niceties are also included.
Kalkulator has been evolving on various OS platforms for the last 15 years, being refined and polished through all this time, and its author has more than 25 years of active experience in numerical methods and software development.