This app simulates the trajectories of nearby asteroids that are potentially dangerous to Earth. You can watch in real time where they are, were, or will be at any given time.
All trajectories of the asteroids shown here are based on real data from NASA's Near Earth Object Program:
http://neo.jpl.nasa.gov/

This free version contains 50 asteroids, including the following:
- 1981 Midas
- 2101 Adonis
- 2135 Aristaeus
- 2201 Oljato
- 2340 Hathor
- 3200 Phaethon
- 3361 Orpheus
- 3362 Khufu
- 3671 Dionysus
- 4034 Vishnu
- 4179 Toutatis
- 99942 Apophis

Besides the asteroids, you can also watch the following planets:
- Mercury
- Venus
- Earth
- Mars
- Jupiter

Get the full version to watch over 1000 near-earth asteroids. The full version includes all asteroids known by NASA whose trajectory comes closer than 0.02 AU to Earth's orbit, and which size is larger than 50m in diameter.

NOTE: Asteroid trajectories are inherently unstable, so they are difficult to predict far into the future. The trajectories calculated by this app cannot be considered 100% accurate, particularly for dates far into the future.