Tag URI
A friend of mine created a RFC for uniquely identifying an entity. The Tag URI scheme uses a tagging entity, to identify the tagger, this is usually a domain or e-mail address owned by the tagger. Then it uses a date to identify the unique owner of the tagging entity.
It basically allows you to uniquely identify something, through time. I think it's a fascinating concept. Add in a way to correlate other tags which refer to the same entity (as it's a many to one relationship between tags and entities), and there's a lot of intersting things you can do (more on this later...).
The only issue I'm having is finding a use or a reason to use them in the here and now. I don't currently have any tagging needs which need to be future-proofed to this degree.
Ideas?
Anyhow, here are some links for your enjoyment: