I've been using threadable for a while. In how I interact with it, it's a smarter alternative to a Mailman list and/or a more usable and comprehensive alternative to Google Groups (not a plugin). It's basically a standalone email service, and the members of a threadable group have two options: threadable can be a totally invisible layer and the email shows up in their standard inbox as usual. Or they can use the threadable interface, which is a lovely and simple way to see all the threads your group is discussing, split off new topics, create subgroups, and so on. It has many uses for project management, but I haven't interacted with that side of it as much. Perhaps others can comment on that aspect.