The Wikalong firefox sidebar uses a wiki as its backend, to store shared annotations about web pages that people visit (see the screenshot).
Cool idea! Globally shared annotations would probably quickly become impractical, but shared with a reasonably sized group this could be very useful.
bertrand's brain grep 
WikalongExtension – Wikalong Firefox Extension
I wrote about Annotatable Python docs. a while ago. Now along comes WikalongExtension – Wikalong Firefox Extension which is a cool extention for Firefiox to allow any web page to be annotated. This is so cool and so useful.