Gene Smith's book has a good overview of tagging in early "social bookmarking" projects, http://epublishersweekly.blogspot.com/2008/09/tagging-people...

Random is an iOS app/svc attempting to infer "associative ontologies" from navigation patterns, http://www.datascienceweekly.org/data-scientist-interviews/r...

"The app also allows you to connect things freely thus letting you express both your rational and irrational self. There are no universal categories or connections between different things - rather it's about an individual's own "ontology" that's created through usage. The "associative ontology" evolves continuously both through the actions of the individual and other people using the system. "