You have a small area. You mouse over it. An area pops up giving you a zoomed in closer look. This is a jQuery plugin that does it. I'm not going to tell you what you should use it for or elaborate use-case scenarios. Your own creativity can help you there.
It's flexible in many ways, in that the "small", "large", and "zoom" areas are all pretty easy to customize (via CSS). It's inflexible in other ways, in that it doesn't "automatically" work by cloning content or anything like that (which is arguably more flexible), and the HTML structure is fairly rigid.
In the emerging custom of jQuery plugins that benefit from demos, I've put several demos and the documentation for it all in one place.