Metanoodle: Asteroid won't hit earth but how close will Apophis come? Updated: China too. Updated: 2011 AG5 Updated: ten thousandth found

The European Space Agency plans a high speed crash onto an asteroid in 2015. A second satellite will stand by and measure how the trajectory shifts. The half ton craft will strike at about 20,000 mph. 99942 Apophis, a quarter mile across, has a 1:250,000 chance of meeting our earth in 2036, "so it would be useful target practice". Since the lump of rock has maybe ten billion miles to cover before then, a small nudge will add up.
The Daily Mail. h/t Drudge.

NASA has plans to land astronauts on an asteroid within 15 years. 'You can't land on an asteroid because you'd bounce off - it has virtually no gravity. [...]Nasa is thinking about jetpacks, tethers, bungees, nets and spiderwebs to allow explorers to float just above the surface of it while attached to a smaller mini-spaceship'. (Same story, or try this one.)

Russia is thinking of hitting that asteroid too and promises not to use nuclear weapons. Story from 2009.

Just how likely is a hit from Apophis on April 13th 2036?
1:37 chance (2004 estimate reported.)Apophis: The Asteroid That Could Smash Into The Earth
1:250,000 chance (2011 estimate, missing by 20,000 miles)Will Apophis the asteroid hit Earth in 2036?
For detail of all estimates, see Wikipedia.
NASA has a section dedicated to tracking asteroids: Near-Earth Object Program - NASA As of August 14th 2011, 8071 objects have been identified and are being tracked.

Updated: China Reveals Solar Sail Plan To Prevent Apophis Hitting Earth in 2036.
Updated: NASA's "WISE" survey puts tight upper limits on asteroid population. 911 of the estimated 981 over 1 km objects are being tracked. A few surprises show up though.
Updated: 2011 AG5 is a possible hit in 2040. Only 140 meters across, before it hits the atmosphere. Could be used as "proof of concept" to test our ability to deflect incoming rocks.
Updated: Ten thousandth near earth object discovered June 2013.