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.

Class for working with continued fractions

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

Numerical integration for Ruby with a simple interface.

Minimization algorithms on pure Ruby