what is the tallest building in the world?

CN Tower Toronto at 1815 ft

@gooch, technically, that's the world's tallest tower. There are even taller structures (radio towers), but the world's tallest building is Taipei 101 in Taiwan at 1671 feet.

http://en.wikipedia.org/wiki/List_of_tallest_buildings_and_structures_in_the_world

The CN Tower is really cool, though. On the observation deck, you can walk out on thick plexiglass flooring. It really screws with your head. I have no fear of heights to speak of, but I ended up crawling out on the clear glass floor on my hands and knees before willing myself to stand upright. Pretty wild.

the new skycraper in dubai is the tallest

@xgunther it has not yet been completed it is planned to be taller but currently is still shorter

I believe they have already completed its frame, they just need to make it into a useable building.

1758.86ft ..so far , so even incomplete Burj Dubai is tallest

yep, i think dubai's is now officially the tallest even though it's not completed -- i read an article on the day it crossed the line -- here's one: http://news.bbc.co.uk/2/hi/middle_east/6910536.stm

The under construction Burj Dubai in Dubai, United Arab Emirates is the world’s current tallest freestanding structure on land, rising 604.9 m (1,985 ft). When completed, it is planned to rise over 800 m (2,625 ft).

The CN Tower in Toronto, Ontario was the world’s tallest freestanding structure on land from 1975 until the Burj Dubai surpassed it in 2007, rising 553.33 m (1,815 ft). It is currently the world’s tallest completed freestanding structure on land.

http://en.wikipedia.org/wiki/List_of_tallest_buildings_and_structures_in_the_world

Top 20 Under Construction
1 Burj Dubai
As of 24 October 2007, Burj Dubai’s official website reported its height to be 585.7 m (1,922 ft), with 156 completed stories.
http://www.tallestbuildingintheworld.com/building_id_106_Burj+Dubai.php

Top 20
With a massive 60-foot spire inspired by Twaiwan’s native bamboo plant, Taipei 101 Tower is one of the tallest buildings in the world. At 508 meters (1,667 feet) high, the tower and its spire outrank the Petronas Twin Towers in Kuala Lumpur, Malaysia. Taipei Tower has 198,347 m2 (2,336,000 sq. ft.) of office space, 77,033 m2 (796,000 sq. ft.) of retail space, and 83,000 m2 (893,000 sq. ft.) of parking space for more than 1,800 vehicles.
http://www.tallestbuildingintheworld.com/building_id_2_Taipei+101.php