Our content on the Intranet is not tagged and is unstructured. What is the best means to inject semantics?

This simplest way is to use Semaphore alongside the search engine – so that content is classified automatically and the meta-data is stored in the search engine index. This is easy to implement and will make a huge difference to the search and navigation experience and the findability of content. Alternatively Semaphore can be integrated with the underlying content management system – if there is one.