Google Docs
Google Docs (and Spreadsheets and Presentations) is a free application that allows a group of people to collaborate on one document. Unlike a wiki that can be set up similar to a website, Google Docs tend to be used when creating a stand alone document, similar to a Word document, Excel spreadsheet or Google Presentation.
Though teams frequently use Google Docs for collaborative team activities, instructors often use Google Docs as well, when wanting the entire class to contribute to a document.
To learn more about Google docs click here, see both Google Support and Google Docs in Plain English.