number_format.rb

module NumberFormat
  def to_comma_separated
    self.to_s.gsub(/(\d)(?=\d{3}+(\.\d*)?$)/, '\1,')
  end
end

class Integer
  include NumberFormat
end
class Float
  include NumberFormat
end

123123123.to_comma_separated      # => "123,123,123"
123123123.12.to_comma_separated   # => "123,123,123.12"