AnythingZoomer is a JQuery plugin useful to zoom specific areas such as text and images. You can customize the zoom area via CSS and also use multiple zoomers on the same page.
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.