Asteroid 2005 YU55 will pass closer to the Earth than the orbit of the Moon on November 8th, 2011. At over 1300 feet in length, this one would cause some serious damage if it struck the Earth. This is the nearest pass of a know space rock since 1976 and there won’t be another (fingers crossed) until 2028.
2005 YU55 will be able to be seen by a telescope with an aperture of 6 inches or larger, so plenty of data will be collected as it passes.
Of course we can’t mention near earth collisions with giant rocks with out mentioning some 2012 doomsday chatter.