server.rb

require "rubygems"
require "eventmachine"

module Echo
  def receive_data data
    send_data(data)
  end
end

EM.epoll
EM.set_descriptor_table_size(32768)
EM.run do
  EM.start_server("0.0.0.0", 12345, Echo)
end

To run it, make sure you have ruby and rubygems installed. Then run: gem install eventmachine. Finally, save the gist to a file server.rb and run ruby server.rb.