HiDigit
HiDigit is a new calculating software with extended capabilities. This is an essential application for math, algebra, calculus, geometry, physics and engineering students. The main advantage of the software is a simple input format even for the most complicated formulas. ... The compact size of the software does not hinder the performance. ... The interface is straightforward and very easy to navigate through. ... No special skills or knowledge are required. ... To the contrary, the program can even be used by the multiple users simultaneously.
http://www.binarythings.com/hidigit/hidigit.exe - 1Mb - Download - Screenshot

Kalkulator
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,... , histograms, polynomial regression), column operations on stat data, polynomial roots, linear algebra (vector/matrix operations and systems of linear equations). ... There ara also unit conversions, physical constants, computer math (binary, octal, hex), and function graphs included.
http://www.wrotniak.net/works/kalkulator/_zip/kalkulator-inst.exe - 956Kb - Download - Screenshot