Wikalong is both a Wiki and a shared space for web page annotation. Notes can be associated with any URL, so there's one set of notes for http://slashdot.org, another for http://www.google.com/search?q=Wikalong etc.
Wikalong can be accessed from any browser using the Wikalong Redirection tool but the preferred interface is a Firefox extension which shows the notes for the current page in the sidebar. Anyone can edit these notes.
The Wiki itself is stored on a central server: when you put a CamelCaps word (or a word inside square brackets) into a note it is rendered as a link to that page in the Wiki. Anyone can edit that page, too.
The final twist to this interesting social experiment is that a live list of links back to newly-annotated pages is available, both as a web page and as an RSS feed.
Everything you need is at http://wikalong.phunnel.org/wiki/