Harry Chen posted some useful commentary about Tom Gruber's efforts to bring structure to tagging and to foster interop between the tags of different apps.

In order to extend what is possible in the social web, tags need to be recognized for what they are - plain literals associated with a subject through the slippery predication of "I tagged." I tagged, but what did I mean? Without too much work, community developers can get started now on making tags in their apps more meaningful.

First, apps can allow for the entry of tags for a particular subject through different predicates. If users can be trusted to type in some interesting tags, why not offer tagging through a context? For example: a photo-sharing app that encourages users to add keywords - separately - for interesting aspects of a photo such as "contains", "has colors", and "reflects cultures".

With subject-predicate tagging it will be possible to offer all sorts of interesting aggregations and structured search options from what users enter for each pair. This approach also puts the application much closer than "raw tags" would have to being able to publish consumable RDF for use by other sites.

Second, in conjunction with offering tagging through predicates, a community culture can be fostered around the tagging activity. This does not have to limit what an individual wishes to enter - it is merely the encouragement of meaning that will make the aggregated-aspects of the system more useful.

Rather than bother with tedious, mouse-centric UIs or complex and limiting text validation, the app can encourage norms for what to enter for each property through a tag cloud of commonly used tags - e.g. colorful adjectives to represent user ratings.

These two suggestions for web developers will add many new interesting dimensions to an online community without asking too much of the developers (or the users for that matter).

I am developing a Rails plugin along the lines of "Acts As Taggable" that will enable users to add free-form tags for any field of a Rails model. Probably I will call this plugin "Acts As Assertable."