A friend of mine once asked me my opinion on the 9/11 conspiracy theories, and I said I didn't think there was much to them, and he said "What about WTC building 7? It collapsed at near free-fall speed into its own footprint, despite not being hit by a plane." and I said "I'm sure you're mistaken, but I'll look into it."

And so I looked into it, and... well, he wasn't mistaken. A 47 story skyscraper collapsed at near free-fall speed into its own footprint, despite not being hit by a plane.

The FEMA report contains the following rube goldbergian explanation for the collapse:

1. Power to the Twin Towers was wired from the substation in WTC 7 through two separate systems. The first provided power throughout each building; the second provided power only to the emergency systems. In the event of fire, power would only be provided to the emergency systems. This was to prevent arcing electric lines igniting new fires and to reduce the risk of firefighters being electrocuted. There were also six 1,200 kW emergency power generators located in the sixth basement (B-6) level of the towers, which provided a backup power supply. These also had normal and emergency subsystems.
2. Previous to the collapse of the South Tower, the power to the towers was switched to the emergency subsystem to provide power for communications equipment, elevators, emergency lighting in corridors and stairwells, and fire pumps and safety for firefighters. At this time power was still provided by the WTC 7 substation.
3. Con Ed reported that "the feeders supplying power to WTC 7 were de-energized at 9:59 a.m.". This was due to the South Tower collapse which occurred at the same time.
4. Unfortunately, even though the main power system for the towers was switched off and WTC 7 had been evacuated, a design flaw allowed generators (designed to supply backup power for the WTC complex) to start up and resume an unnecessary and unwanted power supply.
5. Unfortunately, debris from the collapse of the north tower (the closest tower) fell across the building known as World Trade Center Six, and then across Vesey Street, and then impacted WTC 7 which is (at closest) 355 feet away from the north tower.
6. Unfortunately, some of this debris penetrated the outer wall of WTC 7, smashed half way through the building, demolishing a concrete masonry wall (in the north half of the building) and then breached a fuel oil pipe that ran across the building just to the north of the masonry wall.
7. Unfortunately, though most of the falling debris was cold, it manages to start numerous fires in WTC 7.
8. Unfortunately, even with the outbreak of numerous fires in the building, no decision was made to turn off the generators now supplying electricity to WTC 7. Fortunately, for the firefighters, someone did make the decision not to fight and contain the fires while they were still small, but to wait until the fires were large and out of control. Otherwise, many firefighters may have been electrocuted while fighting the fires.
9. Unfortunately, the safety mechanism that should have shut down the fuel oil pumps (which were powered by electricity) upon the breaching of the fuel line, failed to work and fuel oil (diesel) was pumped from the Salomon Smith Barney tanks on the ground floor onto the 5th floor where it ignited. The pumps eventually emptied the tanks, pumping some 12,000 gallons in all.
   1. Unfortunately, the sprinkler system of WTC 7 malfunctioned and did not extinguish the fires.
   2. Unfortunately, the burning diesel heated trusses one and two to the point that they lost their structural integrity.
   3. Unfortunately, this then (somehow) caused the whole building to collapse, even though before September 11, no steel framed skyscraper had ever collapsed due to fire.

The NIST report says that the failure of a single column near ground level led, first to a vertical progression of failures, causing the collapse of the East Penthouse, followed by a horizontal progression of failures leading to the near-simultainious collapse of all of the building's 27 core columns.

The official 9/11 commission report, in its 568 pages, does not mention building 7 at all.