The only state machine I'll (probably) ever need.

state_machine.rb

class StateMachine
  def initialize
    @transitions = {}
  end
  def transition(opts, &block)
    from = opts[:from]
    to   = opts[:to]
    @transitions[from] ||= {}
    @transitions[from][to] = block
  end
  def transition!(from, to, *args)
    if @transitions.has_key?(from) && @transitions[from].has_key?(to)
      @transitions[from][to].call(*args)
      return to
    else
      raise "Cannot transition from #{from.inspect} to #{to.inspect}"
    end
  end
end