Certainly the Ted Nelson vision for hypertext was all about annotation, in particular as a means of correcting half-truths and distortions in the mass media.

In 1995 Dan Huttenlocher and I created CoNote, which supported annotation on web documents. See Computer Support for Cooperative Learning, 1995 conference, http://jamesrdavis.org/Papers/CSCL95/cscl95.rtf