Development version

The sources can be found in the Nexuiz and Darkplaces SVN repositories, as seen here: SVN Repositories
This version is in constant evolution and can be played on the Over The Lazy Dog servers, and on GreEn's, though the latter is updated less frequently.

You will often find out about the changes affecting gameplay on the Alientrap forums.
The invisible changes can be found in the svn logs of both repositories. See SVN Repositories for more info.