Alfresco looks like an finally usable, open-source and enterprise-grade Content Management System (plus it sounds tasty). It's Java and requires an Applicationserver, but that should be no problem as dedicated Linux servers are now reasonably priced. There are also certified versions with support, warranty and all the bells and whistles some Enterprises care about.
Alfresco does out of the box:

• Enterprise Content Management (ECM)
• Document Management
• Collaboration
• Records Management
• Knowledge Management
• Web Content Management
• Imaging