worker.rb

require 'thread'

class Worker
  def initialize(count = 1)
    @queue, @closing, @threads, @mutex = Queue.new, false, [], Mutex.new
    add_worker(count)
  end

  def add_worker(count = 1)
    @mutex.synchronize do
      @threads += count.times.map { Thread.new { @queue.pop.call until @closing } }
    end
  end

  def run(block = Proc.new)
    @queue << block
  end

  def close(&block)
    run do
      @closing = true
      yield if block_given?
      wakeup
    end
  end

  def join
    @threads.each(&:join)
  end

  private

  def wakeup
    run { wakeup if @queue.num_waiting > 0 }
  end
end

But that rather depends on your use case.

I have an old piece of code that looks awfully similar to this, but uses throw/catch instead of the @closing thing: http://burgestrand.se/code/ruby-thread-pool/thread-pool.rb (I partly did it as an experiment with rocco)

It does not support adding more workers, however.