Collaboration Open Source
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See also:
CCF (Collaborative Computing Frameworks) - A suite of software systems, communications protocols, and tools that enable collaborative work for natural sciences research, "collaboratories".
Collaborative Virtual Workspace - A collaboration software environment that provides a "virtual building" where teams can communicate, collaborate, and share information, regardless of their geographic location.
NCSA Habanero - framework designed to give developers the tools they need to create collaborative Java applications. The framework provides the necessary methods that make it possible to create or transition existing applications and applets into collaborative applications.
Yoga - free software workalike of Lotus Notes Help build the largest human-edited directory on the web.