blog.forret.com: Folksonomizer: generic folksonomy service
Peter Forret proposes a centralised universal metatagging system accessible to sites which aren’t natively tag-enabled.
The centralised aspect makes adding extra layers of service such as relationship mapping an interesting possibility. Combine it with something like tag-stemming and you might have something.
Of course, centralised can also signify single point of failure. Like the recent hacking of Acta Divina which suddenly deprived thousands of sites of their daily statistics.