Social bookmarking, tagging, annotating and note sharing of the web in public or private groups, or just for yourself.
"Our efforts are based on the annotation standards for digital documents developed by the W3C Web Annotation Working Group. We are partnering broadly with developers, publishers, academic institutions, researchers, and individuals to develop a platform for the next generation of read-write web applications. You can follow our development progress on our roadmap. Many have contributed tools, plug-ins and integrations."
https://web.hypothes.is/
https://web.hypothes.is/about/
sourcecode and other projects at https://github.com/hypothesis
https://web.hypothes.is/about/
sourcecode and other projects at https://github.com/hypothesis
A Chrome extension. Firefox and other users will have to use the bookmarklet for now - which works very well
see web.hypothes.is/installing-the-bookmarklet/
run by a public foundation apparently. source code is open but not copyleft, alas.