The MusicBrainz server is not available as an executable software. You have to install the 3rd party products which are required to run the product and then checkout the the modules mb_server from Subversion (See How to Setup a MusicBrainz Server for a step-by-step instructions on how to setup your own copy of the server).
The mb_server is licensed under the GPL (Gnu Public License).
There is a vast number of versions available for the releases the mb_server project has gone through since its beginning in 2000. You'll find the current running version in the lowest right part of any page running on http://musicbrainz.org (version at the time this document was written:
server version: RELEASE-20081123)
The source code of mb_server us available using anonymous Subversion access. Read the Subversion document for instructions on how to use Subversion and checkout the mb_server source code. Alternatively, you can browse the mb_server Subversion repository directly using the source browser.
svn checkout http://svn.musicbrainz.org/mb_server/trunk mb_server