I have been experimenting with online data applications (for example, my DabbleDB FactBook RSS feed and experiments with Metaweb’s freebase.com system).
I am adding Google’s Data APIs to this cornucopia of structured data stores. If you are a Java developer, Google’s GData APIs download is a good quick start.
GData and DabbleDB are great for organizing your own data online and then using that structured data in your own applications. Freebase.com offers a wealth of “other people’s data”, as does DBpedia: Wikipedia information culled as RDF data.