I have a MacBook Air which has a built in camera. Is there any simple application (preferably one that comes with Mac OS X) which simply takes a picture and saves it as PNG or JPG?
Yes, the application is called "Photo Booth," and lives, I'm pretty sure, in /Applications/Photo Booth.app
After you've taken a photo, you can drag an image to the desktop or to a folder in the Finder to save as a jpg image.
ImageSnap is another great command line utility for picture taking or video recording from a connected camera (whether it's your iSight camera or a plugged-in camera). It's a handy little application.
Doesn't come pre-installed with OS X but isightcapture supports command-line picture-taking with the built-in (or external) iSight.