There are a plethora Web 2.0 technologies - each has its own useful aspects. But, none has captured the attention of educators more than the Wiki. Borrowing the Hawaiian word for "quick" - the Wiki enables collaborative document creation oline.
For educators, it provides a solution for the age-old quandry of group projects. Have you ever assigned group project only to be approached afterward and told that one student did nothing, or did the whole assignment? Wiki has the solution - a complete history is recorded - each contribution, each change, each addition, each correction is visible to all.
Of course the Zoho suite http://www.zoho.com and Google Docs http://docs.google.com provide similar features.