KnowledgeTree has a community edition which is free and Open Source. It's a fine product and has many unique features that you won't find in any other OpenSource document management solutions (like the WIndows drag & drop client), but usage often feels kludgy, and its architecture is largely based on deprecated PHP4 code. I set it up for a client earlier this year, but we agreed to turn away from it because we discovered ...
ResourceSpace. It is clearly built mainly for the archival of visual media, but it has so far been working fine for documents as well (with some limitations, see below). It is completely Open Source, based on PHP, and very quick to install. It has extensive workflow features, a very nice-looking interface, and on-line preview of many document types.
The only thing it doesn't seem to be doing is full-text index Word documents - I have a Google Groups question running on the topic. It uses OpenOffice integration to create previews of Office documents which which I'm currently having character set issues (Umlauts breaking in the document preview). I don't know whether these can be fixed.