I came across an interesting 'what-if' article in the New Yorker recently posing that very question. Andrew Sullivan was the main contributor, and posits that 9/11 came at a good time, all things considered, because it warned us and woke us up to the Islamofascist threat.
--George W Bush would not have been reelected; rather, he would have focused almost entirely on a domestic agenda
--Al Gore would be the current US President
--Iraq would not have been invaded
--Afghanistan would have been bombed to take out Al Qaeda training camps
--Al Qaeda would be found to be thoroughly prepared for a devestating simultaeous terror attack on western cities, such as poisonous gases being released on the metro in London, Moscow, New York and Tel Aviv during rush hour, killing tens of thousands.
--Al Qaeda would by now have smuggled nuclear material into certain western countries