Using Online Tools for Collaboration

What is the single most useful online tool for helping students collaborate on school projects? 

How can this tool be used to help students do things differently AND how can this tool be used to help students do different things?

MaryFriend Shepard is a professor at Walden University, and coordinates the EdS and PhD in educational technology programs. Her research interests are in ePortfolios for assessment of standards, and distance education pedagogy.  She has taught 2 - 12th grade, and at the University of Cincinnati, Indiana University Bloomington, Florida State University, and Thomas University. Her expertise is in educational technology, distance education, social studies education, and teacher education.  To contact her: maryfriend.shepard@waldenu.edu or friend@rose.net