The only part of 9/11 that is in any way a conspiracy that's actually plausible to me, the only thing that's at least possible, out of all of the crazy theories suggested, is Bush & Co. finding out via some vague intelligence report that an attack was coming shortly before it happened, whether he found out a couple hours beforehand or a couple days.
This has happened in the past, I think even in America (although I'm not 100% sure, can't remember the exact examples) - Politicians allowing an enemy to strike so that they can rally the country behind them and pass some laws that they want to pass. In this case there could be even reasonable deniability on the part of Bush - intelligence is not always foolproof. You don't act on every single piece of intelligence for this reason.
I really don't see a way for anyone to be able to orchestrate such an elaborate attack from Washington, without anyone finding out about it. What's far more likely is something like this:
Some advisor to Bush: "New York is potentially going to be attacked by Islamic extremists in some capacity in 2 weeks or so, there's all sorts of chatter we've been picking up that hints at it"
Bush: "How dangerous is this?"
{X}: "Seems to just be a moderate attack, maybe a small bomb or a plane being hijacked or something like that, we're not sure, but there's at least 10-20 people involved"
Bush: "Alright, keep a tight lid on this, we don't want to alarm anyone for no reason, you know how bad intelligence can be" and later to his inner circle: "If this happens let's try to pass that Patriot act thing, if not, we'll come up with a plan B. And make sure our jets aren't ready to respond at all, and do it passively so you have plausible deniability."
Out of all the conspiracies ever suggested, this one seems the most plausible to me. And I mean, I don't even think that's what happened, but it just seems like it at least could have.
