I'm not actually certain that what I'm looking for is called project management software. I want something very simple. Suppose three people are working on a project together. The software should give them a place in the cloud to share files; and the collaborators should send messages to the other group members, which can all be viewed in the cloud, in a place associated with that particular project. (Since I might have other projects going with other people.) I should be able to choose a setting that sends me an email when someone uploads a file or writes me a message. The email should contain the content of the message, and the uploaded file should be attached to the email. The advantage of this method over plain old email, and attachments, and a shared dropbox folder, is that if I want to look for an old message, I don't have to sort through all my email -- I can just go to that project online, and see ALL the messages that have been exchanged as part of that project.
It would be a plus if the uploaded files could have tags.