Contents [hide] 1 Purpose 2 Relationship to the Hypertext Web 2.1 Markup 2.2 Descriptive, and extensible 3 Skeptical reactions 3.1 Practical feasibility 3.2 An unrealized idea 3.3 Censorship and privacy 3.4 Doubling output formats 4 Components 4.1 XML, XML Schema, RDF, OWL, SPARQL 4.2 RDF - URI, XML, namespaces 5 Projects 5.1 FOAF 5.2 SIOC 5.3 SIMILE 5.4 Linking Open Data 6 Tools 6.1 Browsers 7 Services 7.1 Notification Services 7.2 Semantic Web Ping Service 7.3 Piggy Bank 8 See also 9 References 10 Notes 11 External links