_why was one of the first to implement this, in 2005. It wasn't a chat, but a comment system for hackers.
The installation instructions were intentionaly elusive:
http://web.archive.org/web/20051125122020/http://hoodwinkd.h...
The IP adresses point to DNS servers that serve two custom domains: hoodwink.d and ___._ (or you could add them to your host file? not sure...).
From there you could browse those sites that had further info, and enjoined you to install a greasemonkey script that injected the comment div in the target pages.
To add hoodwink'd comments to a site, you had to register it at hoodwink.d, with an URL regexp to recognize the site, and an xpath query to locate the point where the comment thread would be inserted.
There were also RSS feeds, that allowed to follow a commenter, or get all the updates of a given site.
Cool stuff.
.
Here is some more info
A longer description: http://ecmanaut.blogspot.com/2006/01/hoodwinkd.html
Magnus Holm, debugging the system: http://oldblog.judofyr.net/posts/who-broke-hoodwinkd.html
A few screenshots: http://www.flickr.com/photos/robbyrussell/3838274563/in/phot...