Best Opensource/Free Document Management System?

mcdade writes: "For 10 years we have been using Lotus Notes to manage documents, data and of course email/calendar/scheduling. It was chosen since it was cross platform and looked the same for both PC and Macs. Now the company has gone purely Mac (including OS X server 10.5). Most things are offloaded on to the mac server that the Domino server would handle (email and calendaring). However there are a number of things that Mac does not do well. There is no way to store/sync documents easily and securely like in Lotus Notes. We already use afp and smb for shares on a server in the office and I have a VPN setup for those users who want access from outside but they complain it's too complicated. They wanted to give up Notes for email and calendaring but seem to want to keep it for secure docs. I really don't want to support the 2 systems and I'm looking to shutdown the 2 supporting Notes servers. Anyone have suggestions for some nice replacement software for Document management and storage. It could also run on Linux/Unix, just not Windows. Now is the time to pitch your new software if you are a developer!"