Ankh: When I was starting to mess around with XML to the point that I wanted to read specs and write programs with it (early 2000, I guess), XLink seemed like one of the more interesting specs being produced. I could imagine a browser of some kind that would allow me to create annotated versions of documents that were otherwise read-only to me. I would just have to create a file of arcs between two external targets -- a point in the source document and the annotation document. I could write the annotation myself or just use this feature to connect up two different pages in a way that was useful to me.
Standard support for this sort of thing would be great. Instead of browsing document A that describes the connections between document B and other places, I could just go through B and have the links available as I go -- much more useful to somebody who works like I do, synthesising a lot between different information sources.