Martin Heller has written a thoughtful technical review of Freebase and the Metaweb Query API in an article titled Freebase, the Semantic Web, and the Metaweb Query API on the Strategic Developer weblog at Infoworld. I recommend that any developer interested in the Metaweb read his review. And if you're really interested, I have three invitations to give away for our alpha test.
At Metaweb, we're making our API as open as possible - here's the documentation. The API allows programmers and web developers to query Freebase via MQL (mickle) the Metaweb Query Language. MQL is not your standard SQL "select user_name from users where ..." syntax because our database allows for a great deal of expressiveness and connections between objects with varying schemas.
If you want to ask questions and learn more, our dev team posts on the Freebase Dev Blog. For executive-level discussions about Freebase and the philosophy behind it, see Robert Cook's blog, Freebasics.
Here's a MJT documentation excerpt.
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Mjt is an HTML templating engine that runs entirely in a web browser. It was built for the Freebase service, but it can be used for many other web services. It is distributed as open source.