Also, how are we defining 'end of the world'?
Are we talking the complete destruction of Earth? The loss of it's biosphere? Or simply the death of all people?
I'm going to arbitrarily assume you mean the death of all people, because frankly if everyone's dead I think you'll be hard pressed to find anyone else who'll give a crap about everything else.
From THAT stand point, I don't see the end of the world happening for a long time if at all. Let's be real about this, as far as these things go we're pretty damn hardy critters, and even extremely massive events have a good chance of leaving SOME of us alive to continue on. To kill all people on Earth would require something of sufficient magnitude to be disrupting ALL life on Earth and despite what a lot of people might think we're not really even close to being able to do the damage that nature itself can do at this point.
Thus, I'd have to say a major natural cataclysm. Probably an asteroid or super volcano.
This is assuming that we're STILL RESTRICTED TO THE EARTH. Once we start having things like independent off world colonies (SPACE IS IMPORTANT DAMMIT) I'm hard pressed to think of many things that could come close to killing all of us. Even a massive and exceptionally thorough space war would still probably leave SOMEONE alive, and once we manage to spread to other stars then as far as I'm concerned our species has 'won' the game of life, and the most that anyone needs to be worried about is the entropic heat-death of the universe.