"Imagine the sensory overload of a walk in the park. Every path shimmers with the flow of humanity. Every person drips with the scent of information: experience, opinion, karma, contacts. Every tree has a story: taxonomies and ontologies form bright lattices of logic. Desire lines flicker with unthinkable complexity in this consensual hallucination of space and non-space, a delicious yet overwhelming sociosemantic experience."
This isn't a "semantic web app," but it's a really good "database with a front end." It's a good example of the kind of app that you could build with the semantic web.
"All too often, a preventable disaster happens not because we didnt have the right data, but because we didnt know how to ask the right questions." It may be just me, but I thought that this line of thought was not carried further.