Michele sent in a link to this amazing collection of pictures of air conditioner supports. That's right. Michele collects pictures of the things people use to prop up their air conditioners! She's got pictures of bowls, videos, books, wood sticks, styrofoam, and a whole bunch of other crap that people have stuck up in the gap on the outside of the window. Now, the common wisdom in our household growing up was that supports are not necessary-- the air conditioner is fine by itself. Of course, we've also seen "The Glasses" episode of Seinfeld 500 times, where Kramer kills a dog with a poorly secured Commando 8:

KRAMER: Commando 8 has arrived!

JERRY: Take it to the window.

KRAMER: 12.000 BTU's of raw cooling power.

(Kramer places the air conditioner in the window)

KRAMER: Installed!

GEORGE: That's it? You don't have to screw it in or anything?

KRAMER: No, just plug it in and the Commando 8 does the rest. (And to Jerry:) I'll seal that up later, right?
ELAINE: Yeah, he's got a real horse face. (Elaine, while looking out the window:) Here, look at this! It's the guy with the dog! (She opens the window and screams:) Hey! You down there! Remember me? I had to get shot because of your stupid dog!

DOG OWNER: Who are you calling stupid?

JERRY: Hey, shall we spit on him?

ELAINE: No no no no, come on, let's go downstairs.

(Kramer enters the apartment)

KRAMER: (Singing:) Oh myyyy papayaaaa.

(The air conditioner wobbles)

KRAMER: The air conditioner!

(Kramer tries to keep it from falling by holding it's cord, but it snaps)

KRAMER: I think it got the dog!

So there you go: if you like dogs, probably best to prop the AC up. We, of course, have a cat, so we don't worry about the airconditioner falling outwards.