A very large asteroid, 308242 (2005 GO21), is making its closest approach to the Earth today.
The approach distance is not as close as other NEO close approaches, at 0.0440AU, which is 17.1 times the distance between the Earth and Moon. However, the asteroid is very large, somewhere between 1.4km and 3.1km in diameter, which is of a size that would cause global damage if it were to impact with the Earth. Luckily such a NEO only impacts every several million years.
The exact point of closest approach is at 18:15 UTC (plus or minus one minute) on 21 June 2012. At this point, 2005 GO21 will be travelling at 13.27km/sec relative to the Earth.
2005 GO21 orbits the Sun once every 239 days, with a distance to Sun of between 0.5AU (closer than Venus) and 1.01AU (just further than the Earth).
Below is an animation in one day steps from one month before to one month after the close approach (Animation made with images from NASA/JPL NEO Program Orbit Diagram viewer)