Aliens
Let's just talk about aliens for a second. Who isn't slightly fascinated or horrified with the idea of a faster, scarier and smarter race of aliens/humanoids/little green men who could potentially wipe out mankind and take the planet as their own? It's a little scary, which is exactly why there are so many movies and books about it. I have read a lot of YA books that have used this premise to kickstart their post-apocalyptic environment. A few books that come to mind are In The After, by Demetria Lunetta, Dark Inside by Jeyn Roberts, The 5th Wave by Rick Yancey or Planet of the Apes by Pierre Boulle. There are SO MANY books about this idea, I couldn't possibly list them all. 
Zombie Virus I realize that some people might be sick of zombies at this point, but I never get tired of reading about an apocalyptic world brought on by an army of shuffling, moaning dead people. What's not to love? *wink* Some of my favorite books are about zombies. Like the Rot and Ruin Series by Jonathan Maberry, The Forest of Hands and Teeth by Carrie Ryan, World War Z by Max Brooks or Warm Bodies by Isaac Marion. I especially enjoyed The Dead by Charlie Higson. The Walking Dead graphic novels by Robert Kirkman are brilliant, too. And let's not forget I Am Legend by Richard Matheson! Zombies will never die, so to speak. There are always authors around to resurrect them. (so many undead jokes, so little time!)
New World Order (OMG!) A lot of post-apocalyptic books begin smack in the middle of a government that has been long established. Books like Divergent by Veronica Roth or The Hunger Games by Suzanne Collins are great examples of this. They include governments that have been established in the midst of an apocalypse (and of course they are always sinister and deeply corrupt!). Another post-apocalyptic series that I am totally in love with are the Books of Ember by Jeanne DuPrau. The first book, City of Ember is awesome. The Giver, by Lois Lowry is another good one.
Unexplained and Mysterious Events (gasp!)There is also the scenario in which readers are left in the dark about the exact circumstances that caused the apocalypse that thrust mankind into the stone age and forced him to rely on cans of green beans for survival. (That was a long sentence, sorry. But not too sorry.) I mentioned The Road by Cormac McCarthy above and his book is a prime example of that formula. We never find out WHAT caused the apocalypse, it simply IS. The circumstances of the end of the world are as accepted and standard as today's society. I like it when it's a bit of a mystery - it gives the reader's mind some room to imagine the end for themselves. Ann Aguire's Enclave and S.E. Bodeen's The Compound makes this work for them, too.
Massive Sickness and Disease And then there was this one. This is probably my least favorite of all of the options, if only because it's so realistic and reminiscent of the Bubonic Plague. I'd much rather read about zombies. But this premise is still pretty interesting, because it's so chillingly realistic. James Dashner's Maze Runner Series is all about a disease ravaging mankind (and I'm a HUGE fan of James Dasher's books), and I've heard that Jacqueline Druga's book, Last Woman, is pretty interesting as well.
Nuclear War/Technological Attack This is definitely a scary one! The onslaught of radiation fallout and bombs going off left and right has been a favorite pick for post-ap authors for years. I like to theorize that The Road's unexplained and mysterious apocalypse was brought on by nuclear war. I know that Gray by Lou Cadle was moving in that direction, along with On The Beach by Nevil Shute and The Last Ship by William Brinkley.
What are your favorite post-apocalyptic books and movies?
