The source code runs on the Apache/PHP/MySQL platform, tested on Linux and Mac OS X. You can find documentation on the developers’ page.
Click here for the latest release.
To get the most up to date code, use subversion to check out a copy of deme repository. The command is
svn checkout http://groupspace.org/svn/deme/trunk deme-trunk
You can also use a graphical SVN client, of course. (e.g. TortoiseSVN or JavaSVN)
Deme is fully open-source, free software. It is currently licensed under the Affero General Public License (very similar to the GNU GPL, updated for the web). Deme is Copyright © 2003-2005 Stanford University.