NMatrix is a linear algebra library for Ruby, written mostly in C and C++.

Ruby/GSL is a Ruby interface to the GNU Scientific Library, for numerical computing with Ruby

Minimization algorithms on pure Ruby

Class for working with continued fractions

Numerical integration for Ruby with a simple interface.

Nerd dice allows you to roll polyhedral dice and add bonuses as you would in a tabletop roleplaying game. You can choose to roll multiple dice and keep a specified number of dice such as rolling 4d6 and dropping the lowest for ability scores or rolling with advantage and disadvantage if those mechanics exist in your game.