Tgethr provides simple email-based collaboration for groups
The cool thing about Tgethr is that it's not really another service to use and commit to. You can respond quickly to the group by using your email client, and all the messages wind up in your inbox. Attachments get saved and linked, so you can download them from the web instead of having them sit in the individual inboxes of people in the group. If you're looking for a simple, sensible way to do email groups, Tgethr is a good bet.