Internet discussion groups (aka discussion lists) have been around for a long time. One familiar type is the academic discussion group which is essentially an e-mail broadcasting system where one subscribes to the list and can then post to everybody on the list simultaneously. Other systems require (or allow) for postings to be read on-line. Some display postings in a "threaded" format that attempts to show which previous series of postings (or thread) any particular posting relates to.
Some (free) systems that support groups (such as Yahoo Groups and SmartGroups) also provide for many other kinds of group collaboration, such as photograph and file sharing, joint databases, polls, surveys, shared calendars and live chat. Yahoo groups now also provides RSS feeds (see the section on RSS in this book) of messages posted to a group - the RSS address is the same as the group's message archive address, just add the extension "messages?rss=1". QuickTopic has a group discussion systems that appears to be simpler than most - e.g. doesn't require registration. Chatzy does something similar, but instantly creates a "discussion room" for text chat rather than e-mail discussion.. QuickTopic also has a very nice Quick Doc Review system for collaborative document review.
Bumplist is an interesting (and whacky) discussion list experiment. It is a list with a maximum of 6 subscribers - when a new person subscribes the first one is unsubscribed, so the only way to stay part of the community is to continually re-subscribe. The purpose is to "re-examine the culture and rules of online email lists", er, or something.
Below are links to -
general material on threaded discussions
material critical of threaded discussions as a teaching tool