Basecamphq.com. Not free - but worth the price since it's so clearly designed anyone with one look can figure it out. It is built for exactly that - group discussion, store documents, and photos automatically display in a gallery. One downside, you can't reply to the email - you have to go to click a link in the email going to the site, which can be an impediment for part time users. Or opensource activeCollab if you're willing to host and maintain it, but the staff time will cost you more. Central Desktop is one step up in price and functionality and includes the ability to reply to the mailing list via email.