everyone hoping for an apocalypse, don't fret! there are still PLENTY of horrible things that can (and statistically should) happen in our lifetime, such as:
-Yellowstone supervolcano eruption: last erupted 70,000 years ago, meaning we're several THOUSAND years overdue for an eruption. a full-scale blast would create an ash cloud that would blanket the earth or at least a large portion, killing off the majority of life, both natural and human, in the americas, and disrupting weather in a way that hasn't been seen since the Ice Age.
-polar realignment: where the north and south poles switch their charges. happens on average about every 450,000 years, last one was 780,000 years ago. wouldn't do much to the natural world, but would wreak havoc on anything artificial that uses the earth's magnetic field. satellites would fall from the sky, compasses would point you in the opposite direction etc. plenty of paranoid nations might see their sats fall out of orbit and assume it was an attack, those with nuclear capabilities might launch a counterattack before anyone knows what happened.
-good ol' fashioned meteor blast: they happen all the time. most of them are small enough to burn up before impact, but just ask the dinosaurs what happens if they don't. NASA currently tracks 4700 such objects large enough and with orbits capable of collision with the earth. while none are deemed an imminent threat, there are nine planets (pluto is a planet, dammit!) casting gravitational shadows around the solar system, all it would take is a slight bump to turn a harmless asteroid into a world killer.
-thermonuclear war: when the soviet union disintegrated, many of their nuclear sites feel under the control of nations whose infrastructures were in ruins, and to this day, not all of their nuclear warheads are accounted for. without the proper launch and propulsion means, a TN attack would likely be a local disaster, however as i said before there are a LOT of paranoid nations out there. franz ferdinand might have been assassinated by a lone nut, but world war i happened all the same.
-alien attack: just kidding. this one's just for the movies. life that would have evolved on another planet would have done so under completely different circumstances, and would therefore be highly unlikely to need our resources or be looking to exterminate us for our planet. any contact with an alien intelligence would most likely be exploratory in nature and unlikely to cause harm to us, just the same as our Voyager probes don't have weapon systems.
So remember, when you're feeling very small and insecure,
How amazingly unlikely is your birth,
And pray that there's intelligent life somewhere up in space,
'Cause there's bugger-all down here on Earth.
Loki/Grendel/Gail/Beowulf/Camilla/River/Bathory/Arwen/Lafayette/Deb/Jack Skellington (soon)