The Near Earth Object Program at the Jet Propulsion Laboratory is currently tracking 1391 "potentially hazardous asteroids." You can even click on them and see their orbits in a cool little java app. Like I didn't have enough to worry about.
Most of them are actually so radically out of sync with our position that they won't be a problem in any of our lifetimes. But it's pretty cool that they're being monitored constantly and we're always increasing the accuracy of the orbit predictions. You can even download the orbital info yourself.
Science is awesome.