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.

http://xmlns.com/foaf/0.1/

#1 - 846,893 references

http://blogs.yandex.ru/schema/foaf/

#2 - 576,208 references

http://www.w3.org/2003/01/geo/wgs84_pos#

#3 - 142,875 references

http://rdfs.org/sioc/ns#

#4 - 48,752 references

http://rdfs.org/sioc/types#

#5 - 39,300 references

http://www.geonames.org/ontology#

#6 - 4,772 references

http://www.w3.org/2002/07/owl#

#7 - 3,764 references

http://purl.org/vocab/bio/0.1/

#8 - 3,057 references

http://smw.ontoware.org/2005/smw#

#9 - 2,553 references

http://www.w3.org/1999/02/22-rdf-syntax-ns#

#10 - 2,169 references

http://purl.org/rss/1.0/

#11 - 2,072 references

http://www.w3.org/2004/12/q/contentlabel#

#12 - 1,869 references

http://web.resource.org/cc/

#13 - 813 references

http://usefulinc.com/ns/doap#

#14 - 798 references

http://www.daml.org/2001/10/html/airport-ont#

#15 - 441 references

http://www.w3.org/1999/

#16 - 430 references

http://www.icra.org/rdfs/vocabularyv03#

#17 - 420 references

http://ontoworld.org/wiki/Special:URIResolver/

#18 - 415 references

http://www.w3.org/2000/01/rdf-schema#

#19 - 402 references

http://purl.org/stuff/rev#

#20 - 319 references