On stackexchange.com, logged in as myself on the "my tag sets" tab, I see a list titled "Or, browse popular tag sets". In each item, next to the "x Views" box, I see a list of tags starting with "All sites". The tags are links, e.g. hgrc links to http://stackexchange.com/questions/tagged/hgrc. However, this resource throws a 404.
The same is true of all the tag links I've tried.