QuickTopic: free bulletin boards (message boards) and collaborative document review groupware: "or any one-topic group discussion, use the QuickTopic free bulletin boards instead of just email! Your messages will be in a private central place, and each of your friends can opt to participate by email or just use the web forum. That's because QuickTopic's super-easy single-topic web bulletin boards are also fully email-enabled: you can get and post messages via email. Use it on your web site too. Over 200,000 served."
Like Yahoo Groups, but I think anyone can participate without registering.