The Rumor Mill
I just came across a very interesting article on Third Voice, an application in the 90s that effectively allowed attaching sticky notes to any web site. Other users of Third Voice could then view these notes while viewing the site.
This sounds like a very interesting concept to play with. Imagine being able to go to a shopping site and call up the opinions of other people who shopped there, not unlike the comments you can make about people selling stuff on eBay. I don't think the anonymous approach would work in anything more but a small company, but with logins and moderation a la Slashdot, this could really be fun. Or at least it'd be fun to implement.
This is similar to the Rumor Monger application I saw last year at ADHOC. Which, incidentally, is a clone of an internal Apple application of same name that supposedly cost an exec his job and marriage.
No, I'm not planning to split up couples ;-)