I'm Ken Krugler, founder of Krugle - a search engine for programmers.
Below is an updated version of my introductory post on the priorart-discuss mailing list.
Krugle is a search engine for code and technical content. Millions technically oriented web pages, plus source code from repositories, and 50K+ project descriptions linked to code.
Right now tags (notes) are free-form. They have a name, a note, and then either a line number range in a file, or an entire file/page.
I've been thinking about more structured tags. One level is just “what kind of tag is it”, as in question, comment, bug, description, etc. This seems more in line with what you describe above.
For very structured at the project level, there's DOAP (I had a short blog about it at http://www.krugle.net/wordpress/?p=38) for describing software projects.
But there's this trade-off between structure and easy of use. The more structured we try to make it, the less tags I think will get created. And the trend w/sites like del.icio.us et. al. is to go very unstructured.