Simon Willison - Google Base is interesting:
“Base is a very interesting product for a whole bunch of reasons. The data model is surprisingly simple on the surface: all items have a title, description, (optional) external URL, a “type” and a set of labels (a.k.a. tags) and “attributes". Attributes are something for tag enthusiasts to get excited by - they’re name/value pairs that are kind of like tags in that you can apply them to anything, but more structured and with a greater level of implied meaning.
[…] There’s definitely a trend towards this kind of loose data model at the moment. JotSpot allows all pages within a wiki to have as many extra name/value attribute pairs as you like (even the wiki body itself is internally implemented as a special attribute), and Ning works along similar lines.”