Heaven's Gate cultists killed themselves because they believed that Nibiru - which is a UFO or a mobile planet or both - was trailing along behind the comet Hale-Bopp, hiding in its brilliant tail. This October, a comet named Elenin will enter our solar system. So I suppose it is not surprising that some people believe that Elenin actually is Nibiru, and that, to coin a phrase, the end of the world is nigh.
The concept of Earth's destruction by collision with Nibiru began in 1995, when a woman named Nancy Lieder claimed to have been contacted by extraterrestrials from the Zeta Reticuli star system. She says that they warned her of an interplanetary collision (or close call that would cause a pole shift, it's not clear which).
At the time, Hale-Bopp was scheduled to arrive in our solar system in 1997. Lieder claimed that Hale-Bopp was a hoax, designed to draw our attention away from the real threat, which she called "Planet X" that would roar past and destroy humanity.
Later, as Hale-Bopp drew nigh (and most people believed in its existence), remote viewer Courtney Brown claimed to have "viewed" Nibiru sneaking in on Hale-Bopp's tail. The rest is history. Sad, suicidal history.
These days the collision with Nibiru is frequently being conflated with the end of the Mayan calendar, which is scheduled for 2012. Some people believe that the Mayans somehow predicted that the world would face catastrophic destruction in 2012, which is why they stopped their calendar there.
(I like to imagine the Mayans being puzzled by this interpretation, then explaining that "No, silly! 2012 is when you turn the calendar over and start again!")
Elenin, the current candidate for Nibiru-like destruction, will pass closest to Earth on October 16 of this year. It will swing by at 21,730,000 miles from Earth, about the same distance as the planet Venus. Certain conspiracy theorists don't believe it, and they in fact believe that the comet's name - Elenin - is code for ELE, which stands for Extinction Level Event (this is why the comet is usually referred to as "ELEnin" on certain websites).
The panic about comet Elenin is nothing new, of course. It seems like every comet that has approached Earth has kicked off a mass panic. Comets have been seen as signs and portents for as long as humanity has been able to look upwards.