The Guardian have a good roundup of ways humanity could be wiped out, they use Sir Martin Rees’ stuff as a basis:
1: Climate Change
2: Telomere erosion
3: Viral Pandemic
4: Terrorism
5: Nuclear war
6: Meteorite impact
7: Robots taking over
8: Cosmic ray blast from exploding star
9: Super-volcanos
10: Earth swallowed by a black hole