I didn't want to say here, but Basecamp. And I have absolutely nothing against them but I wouldn't mind saving a grand or two every year if this can do the same thing and hold up to a dozen or so users.
I'd love that as well. But the killer feature that Basecamp has for me is that you can reply to their emails. Half of my clients have created initial accounts and haven't logged back after that. Yet, they can communicate. If this can do that, I'm switching now!