This is BETTER than group answering sites like Yahoo answers. It allows students and teachers to create groups, post questions and answers AND allows teachers to "Academically Verify" the answers.
An amazing yet simple site for group project management. Share the responsibilities, categorize and make notes, to do's and vote on good ideas. Organize and assign tasks including uploading documents and linking to videos.
My preferred means of collecting and sharing links.
Fully integrates with Delicious -- which had been at the top of my list until their new stacking system which I find . . . well . . . gross.
LOADS of other functionality around groups, discussions, annotations, note taking, sharing -- really well designed and gentle learning curve.