Subnavigation Drawer

The NPR site has an elegant way of presenting subnavigation off its top-level navigation: on click, a drawer opens, displaying a set of grouped links (click to see full-sized image).

To close the drawer, you can either click again on the current option (in this case "programs") or click any of the other top-level options.

This approach is well-suited presenting a large number of options, where a simple drop-down menu (displayed on rollover) isn't practical.