Google sidewiki: contribute information next to any webpage
googleblog.blogspot.com
googleblog.blogspot.com
The barrier of entry was rather high, though. You had to tweak your DNS to point to his own server so that http://hoodwink.d would resolve to a server, then install a Mousehole script (Mh is a custom filtering proxy) or later a Greasemonkey script to make the comments and most of the hoodwink.d site appear.
The hoodwinkd sever refused to serve anything when not accessed throuh the proper URL, and the instructions to get there were intentionaly obscure (just a pair of semi-obfuscated IP adresses for he DNS servers and a few vaguely evocative lines about the concept).
I think you can still find the code at github.
Fun times.
Presumably they're fairly confident in their ability to filter spam and to automatically rank such comments fairly low, but I can't help feeling that this will still be contingent on enough high-quality contributions, ie enough serious uptake. I suppose we'll see soon enough.