Richard Gage and David Ray Griffin like to assert that the collapse of WTC 7 is somehow the "key" to unlocking the "conspiracy" behind 9/11. Because it wasn't hit by a plane the collapse is apparently more suspicious than that of the Twin Towers and the explanation given by some Truthers is that it was brought down by a controlled demolition. They cite the fact that it looks quite like a controlled demolition (which it does, superficially) and also the fact that steel-framed high-rise buildings have never before collapsed due to fire alone.
This video wasn't made by me so please ignore the commentary but listen to the fireman explaining, before the building came down, that it's collapse was inevitable. Please also read the comments made by FDNY staff about what they saw and why they thought the building would collapse. It doesn't seem that they were persuaded by the idea that because a steel high-rise has never come down that it follows that one can't come down due to fire.
This video also explains how the WTC7 building came to be so damaged. While it was not hit by a plane, it was hit by debris from WTC1, or the North Tower. The fires were not small, unlike what Richard Gage alleges (but I believe that NIST has ruled out the deisel tanks as being the cause of collapse).
Richard Gage also cites "sounds of explosions" as a charcateristic of a controlled demolition. Here is Kevin McPadden suggesting that the WTC7 building was destroyed that way:
Apart from the strange implication that the Red Cross (!?) was involved in the conspiracy, the sounds that McPadden describes and the rumble through the ground don't seem to have appeared on this video. What does appear is the fact that the building was expected to fall. Again, no one seemed to have contradicted the idea that a steel-framed high-rise could fall due to fire.
More confirmation here.
See also here and here.
Just for the record, here is the beginning of the collapse of WTC2 as well. Notice that the steel frame does give way and its collapse is not initiated by any explosives: