In December 2004, a large asteroid was discovered that will come dangerously close to Earth. In April 13, 2029 (a Friday), this asteroid will be closer to Earth than our geosynchronous satellites. It will not hit Earth this time around, but there is enough uncertainty in our projections about the asteroid's orbit that there is a 1 in 45,000 chance that it will hit Earth during its next passing six years later on April 13, 2036 (a Sunday). Because of this asteroid's orbit, it was named Apophis, which is the Greek name for the evil Egyption god of darkness. Even if Apophis does not hit Earth in 2036, we still have to worry about it possibly hitting Earth every six or seven years when it again passes close to Earth.
If Apophis hits Earth, millions of people could be injured in its impact size of thousands of square kilometers. However, an impact would be unlikely to have a long-lasting global effect.
As usual, Wikipedia has extensive information about Apophis in general and its chances of hitting Earth.
I learned about this after watching this FORA.tv video with Neil Tyson. You can skip to chapter 3, where he begins talking about Apophis, and he finishes talking about Apophis when he moves onto another subject starting in chapter 8.