This page is keep track of collaborative filtering related sources I've found on the web. Unfortunately, I have lots more that I haven't added in here yet.

As you can see, some of these are actually filtering systems, and some are marketing systems for tracking user preferences and patterns. Like it or not, I think both of these are part of collaborative filtering systems. Collaborative filtering contains inherent privacy issues. By exposing and recognizing them, we can hopefully build systems that benefit both content providers, professional searchers, and information/product consumers.

Deja News

 
How to avoid being entered in the Deja News database (from the Deja
News FAQ) [will also prevent Alta Vista from indexing your postings -dam] :

Q: I don't want my Usenet articles to appear in Deja
  News. What do I do? 
A: We have implemented a feature whereby if your article contains an X-Header looking
   like 
      X-No-Archive: Yes
   your article will be excluded from our database forever. 

An example search for an author.

http://smithers.dejanews.com/cgi-bin/dnauthor-profile.tcl?author=ajw@cs.cmu.edu%20(Andrew%20J.%20Willmott)

Other random places

• http://www.interse.com/ Marketing software for tracking web accesses to a site. Uses inference rules to cluster multiple requests into interaction sessions between a single user and the web site allowing site owners to count visits as well as raw requests. Provides formatted survey reports. Uses IP address indices (probably whois) and zip code database to convert source addresses and browser supplied information into geographical area survey. (Does it do reverse finger? probably not)
• David Karger's Homepage with copies of the PARC Scatter/Gather papers. A potential area for collaborative filtering work.