Shiftspace

Shiftspace is a layer that sits above the sites that you visit that lets you highlight text, embed video, leave comments, and create link trails without editing the original page. The site says:

Having wandered for years in an owner-centric Cyberspace, where do we turn for online public spaces? ShiftSpace (.org) seeks to provide a new town-square built above the existing privatized hyper-mall of information that is the World Wide Web. We are building an open-source meta-layer above the web and we would enjoy your company.

http://www.shiftspace.org