http://www.discover.com/issues/oct-00/features/featworld

Natural Disasters:
1 Asteroid impact
2 Gamma-ray burst
3 Collapse of the vacuum
4 Rogue black holes
5 Giant solar flares
6 Reversal of Earth's magnetic field (2012)
7 Flood-basalt volcanism
8 Global epidemics

Human-Triggered Disasters:
9 Global warming
10 Ecosystem collapse
11 Biotech disaster
12 Particle accelerator mishap
13 Nanotechnology disaster
14 Environmental toxins

Willful Self-Destruction:
15 Global war
16 Robots take over
17 Mass insanity

A Greater Force Is Directed Against Us:
18 Alien invasion
19 Divine intervention
20 Someone wakes up and realizes it was all a dream

http://www.thebulletin.org/index.html

#17 is my fav, and it already happend!