Well, Nibiru is certainly not on a collision course with Earth but there are several NEOs (Near-Earth Objects) which are tracked by NASA. These have a minor chance of hitting earth. One of the most popular, perhaps, is the 99942 Apophis.
Here's a list of the NEOs which are listed as being a possible threat to earth: Sentry Risk Table
Actually Apophis, if it crashes on earth (knock on wood) would release sulfur dioxide in the atmosphere and pollute the air that we breathe and mass extinction would occur... Just like what happened to the dinosaurs!