At 412 metres, the twin towers weren't the tallest buildings in the world. The Petronas Twin Towers in Malaysia are (at the time of writing!) the tallest at 452 metres. They are just larger than Chicago's Sears Tower by virtue of their (unoccupied) spires. In fact, the highest occupied floor of the Sears Building is 200 feet higher than the highest occupied floor of the Petronas Twin Towers.
However, China is planning to build something even bigger in Shanghai. Oblivious to the threat of terrorism, the Chinese will build a financial centre 466 metres high. It will take the shape of a slender square prism tapering to a thin apex and features a large hole to relieve wind pressure.