Alien Invasion (Ok it's a long shot)
Massive release of methane from the sea floor.(My Personal favorite)
Super Volcano (Ok it will just off much of us but in a couple of thousand years you'll have good farm land)
Massive Sun Spots (Big ice age)
Gamma Radiation (Another Long Shot)
Nano robots running amok
Genetically Modified Flu and or Small Pox (Super bug?) (Ok no planet killer but it will give somebody or something else a chance to crawl out of the slime and in a million years or two have a shot at the brass ring)
You cannot win a bet on the end of the world. You're either wrong and lose your money or you're right and can't collect your winnings.