There is a widespread and accelerating move towards increasing use of on-line access, both for traditional library materials and existing on-line materials, as well as newly emerging information genres. One of the key problems is providing interfaces that make it possible (and even convenient) for people to navigate this rather chaotic mass of information, both as consumers and as providers.

In the Digital Libraries project at Stanford, we are developing an underlying structure for interoperation of heterogeneous sources, services, and interfaces to the wealth of materials that will form the digital library. In one experiment, which will be described in this talk , we have implemented an experimental system that enables people to share structured in-place annotations attached to material in arbitrary documents on the WWW.

We will lay out the basic conceptual issues, show some prototypical examples of usage, and discuss other experimental usages of our prototype implementation, such as collaborative filtering, seals of approval, and value-added trails. We show how this is a specific instantiation of a more general "virtual document" architecture in which, with the help of light-weight distributed meta information, viewed documents can incorporate material that is dynamically integrated from multiple distributed sources. More information on this project is available on http://www-diglib.stanford.edu/COMMENTOR.