There are so many of these stupid conspiracy theories surrounding 9/11. If you ask me they're all bullshit, but some of them - not any of the ones listed in this thread - are thought provoking.
The US government were clearly not behind 9/11 - but I think it's possible (though unlikely) that a 3rd party, for example Israel could have been behind it. The dancing israelis article is interesting and worth a read - personally I think the attacks were carried out by Al Quaeda, but some of the more coherent conspiracy theories are at least interesting.
But people seem to love 9/11 conspiracies soooooo much, so here's a new one:
The French did it, they're no strangers to terrorism - remember the Rainbow Warrior.