Freebase -- Wikipedia for structured data?

We have discussed varying degrees of data structure on the Internet, ranging from free-text pages like Wikipedia to rigidly structured relational databases.

Freebase is like a structured Wikipedia, combining the advantages of free text and relational databases. Like Wikipedia, Freebase is open -- any user can create and change content. Unlike Wikipedia, Freebase data is structured so users can query it as they would a relational database.

This example illustrates Freebase. "Topics" are instances of formally defined "types." Here the type is person, and persons have formally defined properties like height, weight, and place of birth.

I am adding a new person (a new topic) to Freebase, myself. I was born in Pasadena, California, and when I began typing a value for place of birth, the system quickly showed the "Pasadenas" in its database with their structured descriptions. It knew that the value of the property had to be a location, and immediately displayed names and property values of locations named "Pasadena." This mechanism both saves time and assures that users have a common vocabulary.

Any Freebase user can create a new topic or change the property values of existing topic. For example, you could add yourself (a new person) to Freebase, and you could also change my place of birth from Pasadena, California to Santiago, Chile.

Thus far, Freebase sounds like a fancy relational database. However, qualified users can also change the structure itself. If one is qualified to be a "type administrator," he or she can create new types and modify the definitions of existing types. For example, one could add the property eye color to the definition of a person.

The developers of Freebase hope that, like Wikipedia, it will attract many users, some of whom are motivated to become type administrators. That way, both the content and structure of a domain of knowledge will evolve over time.

If this sounds interesting, listen to this wide ranging interveiw on Freebase and Metaweb, the system used to create it.