Ping the Semantic Web, the repository for RDF documents, has a page that lists every namespace they've come across as well as the number of references to each namespace. I've listed the top 20 namespaces and the number of references to each. It's interesting to see the sharp drop in references once you get past the top 5. Each of the links below point to a Zitgist Browser page for that namespace so that you can explore them further.
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