Here's an interesting interactive visualization I've found that overlays unemployment rates over the geography of the UK. At the different states of the visual, there are instruction tags directing how you are supposed to interact with it.
The most important interaction point is the sliding bar, which allows you to scroll through the unemployment rates for a specific month between June 2005 and April 2010. As you the month changes, you can see the map overall grow a darker shade of color with some areas hit much harder than others, denoted by a shade of purple (colors are explained in the legend next to the map). Worst impacted areas on the map for the month are explicitly listed under the legend. Like the map, this list also dynamically changes as you use the scroll bar.
The visualization also makes use of common-place zoom-able map design: clicking on the map allows you to see specific areas in detail and clicking areas near the edge of the map allows you to pan and a message that points to the "reset map" button to zoom out. The scroll bar is still usable in this state so that you can view unemployment shift for smaller regions in greater detail.
The greatest asset to this visualization is that it uses a continuous interaction style (Spence Ch.5) to present a large amount of data in a small amount of space, even though the data itself is discrete).