I think my favorite web based collaborative technology is Sourceforge (sourceforge.net). Basically, it is a free service which provides source code control, bug tracking and task management to open source projects. What I really enjoy about it is that it provides a number of developers a central place to collaborate asynchonously. Project members can create online forums, mailing lists and assign one another tasks to take care of. At the same time, what goes on can be observed by the community as a whole. My senior project team utilizes this service and two weeks ago some guy in California asked us some questions about the proposed functionality of our project. While it turned out we did (in a way) support the functions he would want, it really helped us understand the needs and terminology of our prospective 'customers'. Basically, the tools to aid a small, low-budget team while providing visibility to the larger community sets Sourceforge apart.