mathtools
A command-driven calculator for numerical analysis, written in C in 2015. You define functions as plain expressions, such as y: x^2+4, and ask it to solve them:
roots of equations with Newton–Raphson
definite integrals with Simpson's 1/3 and 3/8 rules
initial value problems (differential equations) with Runge–Kutta
systems of linear equations by Gaussian elimination, plus matrix addition, subtraction and multiplication
It parses the expressions itself, keeps variables in an AVL tree, and reports the CPU time each run takes. A small web page can send it commands.