Quite opposite to the friendly tagging we've been considering lately, Big Science projects are heading towards carefully engineered categorization and identification frameworks, aiming at interoperability and sharing of raw data. In Astronomy I've already mentioned here (and if not, I should have done so long ago) the International Virtual Observatory Alliance which has defined a standard format for identifiers.
An IVOA Identifier is a globally unique name for a resource. This name can be used to retrieve a unique description of the resource from an IVOA-compliant registry. This document describes the syntax for IVOA identifiers as well as how they are created. An IVOA identifier has two separable components that can appear in two equivalent formats: an XML-tagged form and a URI-compliant form. The syntax has been defined to encourage global-uniqueness naturally and to maximize the freedom of resource providers to control the character content of an identifier.
In Life Sciences domain, the Object Management Group proposes Life Science Identifiers Specification.
This specification addresses the need for a standardized naming schema for biological entities in the Life Sciences domains, the need for a service assigning unique identifiers complying with such naming schema, and the need for a resolving service that specifies how to retrieve the entities identified by such naming schema from repositories.
Interestingly, LSID uses urn schemes, but specifies resolving mechanisms.