I heard about this an intriguing HTML Altimeter bookmarklet via Asa (or “Topographic View”, as the author calls it). It’s a bit hard to describe, but it essentially reassigns the CSS background for each element on the current page to represent the number of nested elements at that point.
Level zero starts at black and each additional nested element gets closer to white (with 16 shades in total). To make use of the bookmarklet, visit the author’s page and drag the “Topographic view” link to your Personal Toolbar. Then, just click that link to see the effect applied to the current page.
As you might expect, well formed CSS-based sites barely get out of black, but table-based layouts can easily approach pink (the color designating nesting past 16). All the same, though this bookmarklet is good for an easy laugh at the expense of poorly built sites, I’m not sure if it has any practical development applications :-/.