snippet.rb

class FiberGroup
  attr_reader :remaining
  
  def self.run(&block)
    new(&block).join
  end
  
  def initialize
    @remaining = 0
    @fiber = Fiber.current
    yield self if block_given?
  end
  
  def join
    Fiber.yield unless @remaining.zero?
  end
  
  def add(proc = nil, &block)
    raise ArgumentError, "block or proc required." unless proc ||= block
    @remaining += 1
    Fiber.new {
      proc.call
      @remaining -= 1 # DO NOT MOVE UP ;)
      begin
        @fiber.resume if @remaining.zero?
      rescue FiberError
      end
    }.resume
  end
end

Example of usage: http://gist.github.com/542772