Nuclear war is a sexy candidate, but I don't believe we will ever choose to self-detonate so freely, and even then, while nuclear war could well end civilization as we know it, humans as a species would carry on. I don't see us ever going extinct so long as this planet is capable of supporting life. We're just too darned adaptable. We will only cease to be in the case of a major cataclysmic geological/cosmic event, such as a collision with a large celestial body (asteroid, comet, moon), if we get baked by a sudden burst of cosmic radiation, or if something really wacky happens with the sun. Anything else might be bad, might be reeeeally bad, but somewhere somehow humankind would soldier on.