activeCollab - web-based, open source collaboration and project management
Web-based project management seems to be all the rage lately, and activeCollab is a great example. A 100% free and open source project that is under 'heavy development', activeCollab 0.6 offers what seems to be the now-standard array of messages, tasks, projects, milestones and documents, along with a tagging system and per-user project permissions. activeCollab's site even includes a list of upcoming features and offers screenshots of a forthcoming 0.7 feature: a much-updated files section.
Of course, this app offers a few advantages over popular web-based project management solutions like Basecamp, including the fact that you can host activeCollab yourself, on your own domain.
If you're interested in learning more about everything activeCollab offers, they offer a helpful screencast and writeup to get you more up to speed without having to install and tinker with it yourself. I personally haven't used this yet, but my web host recently added it as a one-click install, so I'll post a review in a week or so once I move my sites over and have time to get things up and running. In the meantime, why not check out activeCollab for yourself?