Scientists simulate jet colliding with World Trade Center: A very high-quality, five-minute simulation of exactly what happened when one of the planes hit one of the towers.
Found via this CNN article, which says:
The 3-D animation, part of a Purdue study that took two and a half years to complete, will hopefully help engineers design safer buildings, researchers said.
Also, interesting note on why they chose YouTube:
Researchers decided to post the simulation on the popular Web site because of the animation file’s size, which could not be adequately supported by their servers, he said.