Wikalong is an extension for Mozilla Browser/Mozilla Firefox which adds Web Annotation. Each annotation is a Wiki Page (it uses Kwiki Kwiki).
This page has a Wikalong annotation as example for bridging conveniently different wiki-communities. (hopefully someone adds more content here).
A cross between the original concept of Web Annotation (Annotea, as in Amaya's private and public annotations) and wiki technology. The public tracking wiki (en.wikalong.org 
) is an intriguing way to discover new reading by leveraging the assessment of others (worth commenting, maybe worth reading). -- Bo Leuf
Quoting from www.eschoolnews.com :
"WIKALONG - A Wiki-margin for the Internet By Steve Burt on January 12, 2005 in Emerging Technologies.
"2005: Year of the Wiki? Well... perhaps wikis won't quite take off this year to the degree that weblogs did in 2004, but there is not doubt in my mind that you will be hearing much more about wikis in the months to come. If you are not familiar with wikis, they are open-editable web pages. That is, pages which you can edit the content by simply clicking on one button (usually an 'edit' tab) from within your browser. Take a look at Wikipedia to find out a bit more.
"If you want to begin experimenting with Wikis right away and you have Firefox installed on your machine, then I'd urge you to visit Wikalong.org and download the wikalong extension. This extension embeds a wiki in the sidebar of your browser, indexed off the url of the current page you are visiting. That is, wikalong builds a 'parallel' wiki for any page you visit. Thus, you can comment, discuss and wax philosophical about any issue on a page you are visiting.
"In terms of how you and your students could use wikalong, on the simplest level it can be used as a running commentary or parallel blog to whatever page you are looking at. Of course, you can embed links to point to other interesting pages. Simple note-taking or really anything else that you might use a blog, wiki or discussion board for. In addition to the panoply of possible uses, the other strength of wikalong is that you-as-the-user don't have to install anything on a server or rely on any IT support beyond having Firefox and knowing how to surf the Web.
"Tim Lauer originally pointed me towards this tool so as you get it installed you'll see a bit of commentary between he and I on some of these pages. You'll notice that Wikalong has a log-in feature which allows posters to connect with each other."
See original on c2.com