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)
Hypothes.is is an open source project run by a non-profit but I still have concerns about data ownership (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 archivebox doesn't seem to 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.