Google Docs: allows you to author and share documents, spreadsheets, and presentations online (Free, web-based)

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MediaWiki: wiki software originally used with Wikipedia (Open source, requires web server such as Apache or IIS)

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NoodleTools:  teams work collaboratively in real-time on an interactive tabletop to take notes, cite and annotate sources, then outline and write a paper. Multiple instructors can monitor an individual's contributions to the project and give in-context feedback that is visible on students' dashboards. (commercial, web-based)

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PB Wiki: wiki supporting multimedia plugins, tagging, access controls, etc. (Free, web-based)

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TiddlyWiki: "a complete wiki in a single HTML file. It contains the entire text of the wiki, and all the JavaScript, CSS and HTML goodness to be able to display it, and let you edit it or search it -- without needing a server" (Free, cross-platform)

TitanPad: collaborators write and edit a document simultaneously ; EtherPad re-born (Free, web-based)

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TypeWith.me: simple online collaborative writing tool powered by EtherPad; edits by each co-author are assigned a color; can revert to previous versions of document (Free, web-based; via LifeHacker)

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Wiggio: web-based collaboration platform that supports messaging, web meetings, shared calendars, polls, project management, and shared files.  (Free, web-based)

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Wikidot: wiki-building site: "Users can edit content, upload files, communicate and collaborate" (Free, with Pro accounts available; web-based)

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Writeboard: "shareable, web-based text documents that let you save every edit, roll back to any version, and easily compare changes."  (Free, web-based)

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Zoho: suite of online applications including email, document authoring, notetaking, presentations, spreadsheets, etc  (Free, web-based)

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Etherpad: "A Realtime Multiplayer Notepad in your Browser...lets people collaborate on text in really real-time" (Free beta and pending commercial version, web-based)