Since all major alternatives have been covered, I'll just try to focus on my interpretations of some of them:
Technology - while I agree with GTASWC on most of the things he said in this regard, I think, however, that if humanity is put down by technological creations, it will not be by a computer that has become self-aware, or has decided that humans are inferior and need to be exterminated. Instead, I think that a more likely version is that we be destroyed by a machine with a bug in its software. Sounds funny? Well, think about it - so far nobody has managed to create a completely bug free software and, actually, nobody is really trying to do it. What programmers do (and I speak as a student of IT and programming languages in particular) is try to minimize the possibility of a bug occurring.
Wars - well we are pretty much headed to this outcome with all the religious fanatics and the so-called "War on Terror", though I'm not up to date with the US government's current views and plans in this regard. I do remember hearing something about Obama being against further pointless wars, but I'll leave this to be discussed by people more informed than me.
Disease - well, with AIDS still being incurable, homosexuality being considered a lifestyle and sex being used in everything, because "sex sells", I guess this is a very possible way of ending humanity.