OS X 10.9 Public Sources, a mirror of source code from http://www.opensource.apple.com/source, downloaded in late 2013.
This repo is quite large by itself, and you might not want all of it. To grab a specific project, use svn:
svn checkout https://github.com/nneonneo/osx-10.9-opensource/trunk/<projectname>
e.g.
svn checkout https://github.com/nneonneo/osx-10.9-opensource/trunk/zlib-53