When I implemented an experimental Wiki integration with CAREO, I imagined it would be simplest to just tie a Wiki page to each URL, on demand. By extension, that model could have provided a Wiki page for every URL on the internet.
Wikalong does this for all pages in Firefox - promising to become a public, shared margin for the internet.
I'm not a Firefox user (partial to OmniWeb 5 or Safari) but this sounds very cool. Kind of like a Wikipedia for the web...
UPDATE: Just installed Firefox 1.0, and the Wikalong sidebar. That's freaking AWESOME! I think it's probably got some serious room for vandalism - had flashbacks of ThirdVoice - the IE plugin that let you leave notes on pages for others to see. First thing that happened with that one was the defacing of Microsoft's website - only visible via the plugin. This seems more benign - notes are only in the sidebar, not mingled with page content. This could have some excellent implications for teaching/learning with online stuff... Mods of the Wikalong plugin to use a private or institutional wiki for courses etc... Going to have to look into that...