I had an attendee at the Richmond Code Camp ask about Open Source Document Management solutions during my DotNetNuke talk.  That isn't something that I'm at all familiar with, but today during the NTEN Office Hours Program chat hosted by Michelle Murrain, these links came up as things to take a closer look at:

KnowledgeTree: Document Management Made Simple

OpenKM (this one looks the most interesting to me)

Desktop integration for Plone

PDF Import extension for OpenOffice.org - not a DM solution, but a utility that I could have used in the past - very good to know about!