Alfresco vs Plone

Alfresco and Plone are popular open-source enterprise CMS solutions. Plone is easier to set up fairly complex workflows. Besides there are lots of plugings for workflows, and you can make your own quite easy
Alfresco is a Free enterprise content management system for Microsoft Windows and Unix-like operating systems. Alfresco includes a content repository, an out-of-the-box web portal framework for managing and using standard portal content, a CIFS interface that provides file system compatibility on Microsoft Windows and Unix-like operating systems, a web content management system capable of virtualizing webapps and static sites via Apache Tomcat, Lucene indexing, and Activiti workflow. The Alfresco system is developed using Java technology. Alfresco open-source alternatives are Nuxeo, Liferay Portal.
Plone is a free and open source content management system built on top of the Zope application server. In principle, Plone can be used for any kind of website, including blogs, internet sites, webshops and internal websites. It is also well positioned to be used as a document publishing system and groupware collaboration tool. The strengths of Plone are its flexible and adaptable workflow, very good security, extensibility, high usability and flexibility.
Face to face in the news:

2008 - Alfresco Share - social collaboration tool on top of ECM to compete with Plone

Alfresco Share

Alfresco, the open-source ECM system is going to compete with Microsoft Sharepoint. The new version Alfresco 3.0 features the Web interface designed for collaboration and social interaction Alfresco Share. Its capabilities are very similar to Sharepoint: Document Library with a comments and tags, people search in addition to content search, Intranet / Extranet workspaces, activity streams, profiles and customizable home pages, calendars, wiki, blogs and forums. It also provides integration with MS Office via support of Sharepoint Protocol. In addition to the new collaboration tools the new Alfresco includes lightweight development tool Surf for designing user interfaces and automate them using Alfresco Web Scripts.