Hypothes.is
Hypothesis is a web annotation tool - you can annotate any page and your comments are then public for others to see (or you can privately annotate stuff)
Data Ownership
Hypothes.is is an open source project run by a non-profit. They consider all annotations made in public on their platform to be creative commons public domain so it seems like they're unlikely to try and do a runner with public data. However, I still have concerns leaving all my annotations in their silo (What if their service goes down and all my annotations disappear overnight?)
Annotating Archived Content
It's been pointed out in a couple of places that hypothes.is can be used to annotate data in the internet web archive but what about in my own archive?
Using via.hypothes.is with my own archivebox instance doesn't work but it seems that both the official chrome plugin and the unofficial firefox plugin do work with archivebox if you click through to the single page html you want to view before you activate them.
Active Hypothes.is Users
Ton Zylstra keeps a list of active and semi active h users
Interacting with annotations from other sources
Matthias 'x23' Melcher has built a drag and drop thinking/note mapping tool with import and export functionality from a number of sources including hypothes.is, Kindle and Zotero.