This is my cleaned-up version http://stackoverflow.com/a/7749613/28128

standard_deviation.rb

module Enumerable
  def sum
    inject(0, :+)
  end

  def mean
    sum / length.to_f
  end

  def sample_variance
    m = mean
    sum = inject(0) { |accum, i| accum + (i-m)**2 }
    sum / (length - 1).to_f
  end

  def standard_deviation
    Math.sqrt sample_variance
  end
end

note that this is sample stdev, not population -- should prolly name it explicitly.