the following serves as a list of ways the world as we know it could end:

The most common way that people are familiar with is deities intervening
in divine judgment.  The book of revelations according to many conflicting
interpretations is best source of this - although at least half of religions have a tenent of "God coming down and kicking ass big time".  I've heard some cute quotes involving this:  "Jesus is coming - everyone look busy" and "Jesus is coming - and boy is he ever pissed"  There have been many cults and other religions that have gotten all excited over false end of the world claims including:
cargo-cults,  Native American Ghost Dancers, Jehovah's Witnesses, David
Koresh, etc.

Growing up, my generation was rapt in fear of the spector of global
thermonuclear warfare.  Unlike predictions of some, this would have
wiped out many species but not destroyed our planet or species.
Unfortunately, our current level of ignorance and genetic programming
toward violence and the elimination of competing groups makes
holocausts a likely in our future.

Asteriods.  If you saw Deep Impact or Armagedeon in 1998, you get the idea. This case is an example of scientists and engineers armed with technology could save our asses.
Most people see a big Malthusian threat of people destroying the
earth via overpopulation and consumption of resources.
Actually before mankind had science and technology, many
of the great civilizations were wiped out by over-grazing,
elimination of tree cover and erosion.  The lot of mankind
is threatened by some of the following self introduced,
MINI APOCALYPSES:
   salienation of irrigated crop land - water from rivers or underground
which is unnaturally spread on fields, evaporates and leaves behind
salt residue.  You can't do massive irrigation in dry hot climates and
get away with it forever.