I spent part of 9/11 arguing with some 9/11 conspiracy kooks. I didn't waste a whole lot of time because it is a descent into madness.
It's the fault of the Jews
Antisemitism is an indelible stain born by all of Christendom, and "The Jews were behind 911" is just another sour note on the scale of a millennia-old theme.
The "Mossad did it" theory has Jewish men filming before the attack and then dancing in jubilation when the destruction occurred. It's not even worth wasting space on the supposed details. Various disconnected events are hodge-podged together to make it all fit.
Plausibility and Motive are Not Proof
Could Israel have been behind the attacks? Sure, anything is possible. The supposed motive is that Israelis would benefit from us feeling the sting of terrorism that they know all too well, causing us to sympathize with them and subsequently go on a global jihadi safari, killing all their enemies for them.
Sounds nutty, but plausible. The problem for the wackadoos is that plausibility and motive are not proof. If you're interested in a reasoned analysis of this nutball conspiracy, this Wiki at 911Myths.com does a good job breaking it down and exposing its holes and internal inconsistencies.
Let's Try Out Some Liberal Logic
Washington bureaucrats, especially Democrats, profit from this recession. It increased their power and allowed them to worm government into more corners of society. Millions depend to democrats to ladle out benefits and special monetary favors as "Stimulus." Ergo, the Democrats purposely crashed our economy to increase their power.
George Bush Did It
Could there have been a US government conspiracy? Sure. But once again, there's no credible evidence, and that's strange in a country where nothing is kept secret anymore.
"Oh yeah, how do you explain Building 7?! Huh? Huh?"
My simple answer is "I don't have to explain jack! You're the one with the nutball theories, you tell me!"
If you do decide to engage the barking tards, www.Debunking911.com is another excellent resource. I highly recommend it.