9/11 Alternative Theories

So on the 5th anniversary (if you can call it that) of September 11th, I've been asked a lot about alternative theories.

Here is a rather interesting film, Loose Change. The section on the crash into the Pentagon (approximately min. 12-25) is especially interesting. Of course, it makes tons and tons of errors along the way - many of them chronicled at this guide. Conspiracy theorists will love the movie, debunkers will love the guide. Take what you will from either.

One very common theory seems to be that of a controlled demolition. There is a plethora of structural and civil engineering research on this matter; the consensus seems to be, more or less, that "explosives were not necessary to initiate collapse".