If your looking for a rich and powerful document management system, then you cant go wrong with Microsoft SharePoint Service 3.0. There latest edition of this software (which is FREE, and available for X86 and X64 bit system) makes managing information on internet or intranet simple, but yet effective. Easy to setup and manage online libraries etc, this software has now become to benchmark for Fruit Box I.T customers, as we host our customer portal on a sharepoint site. Our online helpdesks are delivered directly into the customers portal, which makes submitting new tasks a breeze.
Some points, firsty you dont have to be attached to an active directory domain to setup users, using local users and groups works as well, however if you have many users you will benefit from joining the server to the domain so you have access to the AD accounts.
I dont recommand installing this on a windows sbs 2003 server as the IIS gets messy, and can make it a real hassle to get exchange outlook web and sharpoint to co-exist as it comes pre-installed with sharepoint 2.0
the handling of security can take a little to get used to, but once you have your head around it, it all makes sense