Wolfram Mathematica (usually termed Mathematica, Mathematica software suite) is a mathematical symbolic computation program.

Calculator provides simple and advanced mathematical functions in a beautifully designed app.

Swift Calcs enables users to effortlessly create and collaborate on interactive computational documents, from any device.

Desmos is an online graph plotting app.

Sage offers business management, bookkeeping & accounting software to meet any business need, from small to midsized businesses.

fxSolver is a math solver for engineering and scientific equations.

SMath Studio is a freeware, closed source, mathematical notebook program similar to Mathcad.

Project Euler is a series of challenging mathematical/computer programming problems that will require more than just mathematical insights to solve.