Cars, sex and music, depending on the audience.
After that, I explain the idea.
Edit: (to clarify) Cars, like you said, because a certain amount of car knowledge is already in the popular mob-brain. It's fairly easy to describe your web server, load balancer and scripting language as either car or engine parts.
Sex is useful when describing 3rd party API interfaces, particularly Facebook. This one can get pretty damn funny when the API is either poorly designed or unreliable. Also if you need to do weird things to make it work.
Music is good for the high-level stuff. Associating the front-end with the vocals, business tier with the guitars, database with the (wait for it) bass, and the server as the drums. There are bands with two or more of any of those roles (and keyboardists, and cow bell hitters, etc.), so this analogy tends to hold up very well as long as you avoid the details.
