I used to work with hummingbird now opentext.com and timematters with worldox.
Indexing is also a hugely important part of a DM.
Maybe have a read of wiki.alfresco.com/wiki/Document_Management
Alfresco offers true Open Source Enterprise Content Management (ECM) - Document Management, Collaboration, Records Management, Knowledge Management, Web Content Management and Imaging.
Web Content Management
Alfresco uses best-of-breed Open Source technologies such as Spring, Hibernate, Lucene, modern standards such as JSR-168, JSR-170, Web Services, Java Server Faces and contributions from the open source community to get higher quality software produced more quickly at much lower cost.
Alfresco is built by a team with 15 years experience in this space, including the co-founder of Documentum, VP of Web Content Management of Interwoven, and other senior industry players.
Alfresco allows users to access a simple to use, Intelligent Virtual File System from their native application or portal with the full power of an Enterprise Content Management System.
Also having plugins for zarafa is another plus