Group items tagged

Take or create an online course (many are free) where you can go at your own pace, post questions and have discussions. Upload many types of files.

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.

Sign up for the free features for teachers. You need to get approval from Diigo but it adds functionality.

A place to have a debate that is moderated and allows the audience to "vote up" questions or ideas.