Alfresco - Open source ECM solution

Alfresco is....

Alfresco is an open source, open-standards content repository built by the most experienced content management team that includes the co-founder of Documentum. The Alfresco product has a lean, modular component architecture that allows new functionality to be added without any system disruption and is significantly faster than proprietary commercial systems.

J2EE-based professional portal projects that require a content management system at a much lower cost than commercial systems.

Low-cost, open source alternative to Microsoft SharePoint™.

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I think that SharePoint definitely has hit mainstream since other vendors/open source products use it as a point of reference ;-) ...
sharepoint