In case you haven’t seen it, Basecamp is a website for project management, but more to the point, it is quite possibly the best collaborative organizational tool ever. Create a project, add the people involved and then share to-do lists, milestones/goals, post messages with files and write on virtual whiteboards, all while assigning everything to specific individuals and keeping everyone current by email notifications and RSS news feeds.
and almost everything is completely free…
The key is in the perfection of its user interface and deceptively constrained feature list. It doesn’t have 4076 buttons, options and gantt charts that you’ll never use. It just has exactly what you need to collaborate beautifully.