What is so funny about this is all the conspiracy theories that sprang up, when the actual official story is one of the biggest and most successful conspiracies of all time. I mean look at it - 20+ foreign muslim extremist terrorists train for months, hijack multiple passenger jets, and demolish one of the largest building complexes in the USA and the world, killing thousands of people in the biggest terrorist atrocity in human history. Er, correct me if I'm wrong, but that looks like a pretty big and dastardly conspiracy to me! But apparently, this is not conpiratorial or dastardly enough for the truther movement!
There are simple principles for how to assess claims and evidence. For example, David Hume's rule - no testimony is sufficient to establish a miracle, unless it is more likely that the miracle happened, than that the testimony were mistaken. You can modify that slightly to apply it to conspiracy theories - no theory is sufficient to establish a conspiracy, unless it is more likely that the conspiracy really happened, than that the conspiracy theory is mistaken.
If you look at the weight of evidence, the official story clearly has the most to support it. The conspiracy theories are mostly riddled with errors, logical mistakes, incorrect assumptions, or rank speculation with no supporting facts. We cannot be certain that the official story is true, and that there are not other secret facts waiting to come out, but on the current evidence, it is the most plausible.
There are other trader heuristics in favour of it too. If someone could uncover a real additional 9/11 conspiracy or cover-up, they would instantly become the most famous journalist on the planet, they would make tens of millions of dollars, get to fuck some of the hottest women on earth, and be able to interview and investigate any story out there, with groups and individuals queueing up to help them and give them acclaim and support. Given those huge incentives, and the fact that every investigative journalist and media group on the planet has spent 9 years looking for this, the fact that NO serious evidence has come out of an alternative conspiracy, highly suggests that it is not actually out there. In today's internet dominated world, with sites like wikileaks, all it would take is one whistleblower, one spy, one reporter, and the whole conspiracy would be blown. So, using trader analysis (i.e. Bayesian probability and human incentives) the odds strongly favour the official story being the most likely to be true.