Thermite

Professor Jones says thermite brought down the towers.

Videos of an apparent thermite reaction in the south tower right before it collapsed here, here and here (warning, this last page takes awhile to load).

I don't think thermite explains everything about the tower collapses (for instance the pulverization of concrete), but thermite was probably used.

I like Webfairy's idea that the demolition of the WTC was sort of like the JFK assassination, where they brought in every shooter from all over the country to be in the area. The demolition of the WTC may well have involved multiple different technologies, from conventional explosives, to thermite, to microwave weaponry.

Conventional demolition simply doesn't explain how all the filing cabinets and elevator doors and doorknobs that existed in the WTC were apparently vaporized.