Fibonize is a project mainly written in Ruby, it's free.
Sample GEM for extending core methods
>> require 'fibonize'
>> 156.closest_fibonacci
=> 144
Build and install the gem $ git clone git://github.com/richardiux/fibonize.gem $ cd fibonize $ rake install
Install development dependencies $ bundle
Running on irb console $ bundle exec irb
Running the tests $ rake