At the most recent TED conference, Stephen Petranek of Discover presented the 10 most likely ways the Earth could end in the near future:
10. We lose the will to survive (depression being the #1 mental illness)
9. Aliens invade Earth (most primitive civilizations do not survive outside contact)
8. Our Ecosystem collapses (e.g. we destroy the marginal tree in the rain
forest or run out of fish)
7. Particle accelerator mishap – when we can make devices powerful enough to make black holes we expect them to evaporate, but we can't be sure.
6. Biotech disaster
5. Reversal of Earth's magnetic field
4. Solar flares
3. Global epidemic
2. Rogue black hole
1. Asteroid collision