"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."
I haven't yet investigate it much. Its strongest features at this point appear to be file based repository and light workflow, and not extensive on the web content management front. Therefore, I am not sure I would yet put it in the ECM space.
