I don't know if anybody has looked at the W3C's annotation server
(
http://www.w3.org/2001/Annotea/) but it looks like a very interesting
for eLearning and knowledge management purposes. You can access some
of the functionality on the client side through Javascript, although
the most robust way to use it is through a client side software
install. (Clients exist for Mozilla, IE, and Amaya.)