The buildings fell exactly as they were designed to fall, in their own footprint.
I watched the towers fall that day, it wasn't instantaneous, it was a progressive collapse. There are any number of videos on youtube that show the towers collapsing. They show the exact same thing that I watched happen live on CNN that horrible day.
Besides, if it was a government conspiracy, why go to the trouble of flying airliners into the towers? Why not just set explosive charges at night when everyone is gone? Sure it'd take weeks to accomplish that, but the effect would be far greater....ie, larger body count. You could still blame it on the terrorists.
Besides all of that, we have tapes of Osama bin Turdhead himself taking the credit for the attacks and he was proud of it.