What online tagging sites are there?

What sites have tagging systems that have maintained their legitimacy over time?

• Answer:

  this is an excellent question. the answer is almost surely that none have, for any length of time, and that unthinkable amounts of work and co-ordination have been in error. 'tagging' is a synonym for categorisation. even pre-internet, I think all massive categorisation schemes have faced existence-threatening problems at scale. the chemical and biological classification schemes, which there have been plenty dictionaries, whose parts-of-speech tags are still non-standard, and feel ridiculous, the dewey system, and the library of congress (facepalm) wikipedia's tagging/categorisation scheme, which many clever people have tried pulling useful information out of to no avail. wikipedia is particularly revealing because it cannot be said to be short on resources, expertise, or patience. Its failure, I think, suggests with force that a tagging, or phylogeny system is not something that goes large-scale happily, and with grace. subject-verb-object systems actually do though, I hope/pray.