While the service is not free in general, everybody can set up accounts for free and run a single project with multiple partners at no cost. If you want to have more than one project at the same time means paying a monthly fee.
Given the current state of glorious absence of anything that could be called an acceptable solution for project management on the Mac this approach is really refreshing. I like the combination of web-based modification of project data and the option of having customized tools for tracking the progress (like using NetNewsWire and iCal on the Mac). Hopefully, posting to the weblogs will come with a XML-RPC interface for weblog clients soon.