It’s not all that useful, but fun to look at. I have a large poster of the London tube on my office wall–as a sort of homage to early environmental information design.
The Web Trend Map is an interesting metaphor based on a subway map. They put some thought into the relationships between entities, even though it’s impossible to represent them on a 2-D map. It’s also interesting how they integrated weather symbols to show forecasts, something you don’t get (and don’t need) with subway map. It’s a mixed metaphor but works nonetheless.