How can I take a picture with my iSight camera? 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.