Alfresco is an open-source solution for Enterprise Content Management (ECM). It competes with products like EMC Documentum, IBM Document Manager/FileNet, OpenText/Hummingbird and many others. The idea is to focus on the content (and not projects or something else) within organizations and that means all kind of documents like Word, PowerPoint, PDFs but more and more also images, sounds and video as well. Increasingly these platforms also set out to handle structured content within databases. My experience is that employing a good information model for the content with an organization is the only way to start bringing some order to all the documents found on file servers.
Anyway, Alfresco is a really interesting open-source alternative to this. It has been designed from the bottom up by the same team that developed Documentum years ago. It features versioning, metadata support, free text searching, workflow management as well as a special web content management addition. The list of open standards it supports is very long and I think that is the right way to go. With some help from my friend Martin I installed it on a Apple XServe running Mac OS X Server 10.4.8 and it looked really nice with some new AJAX-powered user interfaces.