These were generally called metaweb tech when they were popular about 15-20 years ago.
The problem was that everyone had to have a particular author's plugin to see the extra content.
Some of them were better than others, allowing cross referencing, citation insertion, and a number of other sophisticated features.
I don't know if they still are around.
Sites like reddit and hn are really inverted versions of this concept, fixing the basic problem that most sites don't have commentary.
What may be an interesting hybrid is to search the commentary aggregators when you land on a site and then show the hn/digg/whatever comments and permit replying if the credentials are known
The problem was that everyone had to have a particular author's plugin to see the extra content.
Some of them were better than others, allowing cross referencing, citation insertion, and a number of other sophisticated features.
I don't know if they still are around.
Sites like reddit and hn are really inverted versions of this concept, fixing the basic problem that most sites don't have commentary.
What may be an interesting hybrid is to search the commentary aggregators when you land on a site and then show the hn/digg/whatever comments and permit replying if the credentials are known