Rumor Monger was a non-attributed (anonymous) chat system developed by Harry Chesley in Apple's Advanced Technology Group.
Originally, the project was focused on peer-to-peer, decentralized chat, using an epidemic distribution algorithm originally developed at Xerox PARC. The ability to post anonymous messages being just one minor feature. But as usage spread around Apple, it quickly became clear that the combination of easy access (Rumor Monger was left running in the background on user's machine, and was thus just a click away) plus anonymity created a unique social space. Essentially, there was no barrier between thinking something and posting it on Rumor Monger. This led to two things: First, an unprecedentedly low level of "intelligent" content. And second, posting of thoughts that previously would have been kept strictly internal: everything from insults to harassment to company criticism to really bad haggis jokes. (In fairness, at least one group also saw it as a way to brainstorm project design ideas.)
Apple management panicked, seeing visions of harassment suits, and tried to "turn it off." But, being a good, decentralized system, it was essentially impossible to switch off. David Nagel, the head of ATG at the time, and clearly very pissed about the whole episode, asked how such dangerous technology could have been let loose without a way to control it. Management just didn't seem to see any upside to the whole episode. (Had they taken a more enlightened view and pushed instead of pulled, Apple might have beaten ICQ to the chat business by nearly a decade...)
Rumor Monger continued to be used intermittently (with usage spikes often corresponding to periods of layoffs) for years after the main peak of popularity. Some two or three years later, Harry got a call from Apple security asking about it. It seems it was layoff time and someone had posted a message asking if anyone knew of a place you could mail order AK47s. A very valid concern on their part.
The code for this tool was published in CD-ROM, print, and online (ftp.apple.com) in the Advanced Technology Group's nice quarterly journal. Harry also had some Rumor Monger sweatshirts made, and I still have mine. One of the arguments I remember Nagel had was over the name. He did not believe that was the name of the Xerox PARC algorithm. --Steve Cisler
See original on c2.com 
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Harry created a google group for Rumor Monger veterans with two posts in 2005.
"Rumor Monger was a light-weight, broadcast communications system that ran at Apple circa 1990. If you remember Rumor Monger, please post any relevant experiences, stories, observations, etc.
"What would make sense as a conceptual successor to Rumor Monger on the Internet? The same thing on a larger scale? With group-based distribution restrictions? With support for rumor confirmation/voting? Poll Monger?
There was no response until spam started filling the site from 2009 to 2010. All quiet since then.