(99942) Apophis (symbol ) is a near-Earth asteroid that worried people for a short time in December 2004. This was because observations seemed to show that it would hit the Earth in 2029. Other observations had more accurate, or better, ideas. During 2029, Apophis would go through a "gravitational keyhole", an area in space no more than about 400 meters across. This would make it hit the Earth on April 13, 2036. This danger made the asteroid Level 1 on the Torino impact hazard scale until August 2006.
More watching showed scientists that the "keyhole" would be missed. On August 5, 2006, Apophis was lowered to a Level 0 on the Torino Scale. As of October 19 2006 the probability that Apophis will hit the Earth on April 13, 2036 is something around 1 in 45,000. It may also hit in 2038, but however the probability for that encounter, or meeting, is 1 in 12.3 million.
More reading
Cooke, Bill. (2006) Fatal attraction. Astronomy, May 2006, pp. 46-51.
Other websites
Risk assessment
These sources are updated as new orbital data becomes available: http://neo.jpl.nasa.gov/risk/a99942.html (NASA JPL) page and impactor table from NEODyS.
NASA
http://neo.jpl.nasa.gov/news/news146.html (JPL) http://neo.jpl.nasa.gov/news/news148.html (JPL) http://neo.jpl.nasa.gov/news/news149.html (JPL)
Other articles
Closest Flyby of Large Asteroid to be Naked-Eye Visible (2029 approach) http://skyandtelescope.com/news/article_1458_1.asp (Sky and Telescope) An asteroid, headed our way (Christian Science Monitor, on efforts to deflect the asteroid if needed)