For the past few months, I've been using Basecamp to manage projects associated with Wetpixel, and it's been a really fantastic resource. It features messaging, to-do lists, "writeboards" (documents editable by many), milestones, a chat system, and file hosting. In addition, you can subscribe to any of the above via RSS, and to milestones via iCal.
my basecamp site for a new Wepixel project
The service has been perfect for us, and the plan we're on only costs $12/month. We use it for our to-do lists, milestones, file sharing, and collaborative writeboards of all sorts of topics. There's also a free version of the service that doesn't include file hosting nor multiple project support, so you can check out the service without committing any money to it.
If you do check out Basecamp and decide to sign up, we get a small referral fee if you click through here:
Or, you can just enter "wetpixel" in the referral code box when you sign up.