Sire is available under an open source (GPL) license, and is developed on a publically visible subversion repository. You can view that repository by visiting the Google Code project pages for Sire. From these pages you can also browse the source code.
The subversion repository is divided up into two main branches;
These two branches are further sub-divided into branches that describe different versions of the code, e.g. trunk contains the latest stable version, branches/devel contains the latest development version, and branches/users/chryswoods contains the absolute latest development version being developed by the developer "chryswoods". There are also branches corresponding to released versions of the code, e.g. tags/jchemphys07_paper contains the version of Sire used to run the simulations presented in Woods, Manby and Mulholland, J. Chem. Phys. 2008.
This branching format is described in the README file contained in the head of the repository.
To download Sire, use a subversion client to download the branch that you want, or, using the command line, type;
svn checkout http://sire.googlecode.com/svn/corelib/trunk Sire
to get the "trunk", or type
svn checkout http://sire.googlecode.com/svn/corelib/branches/users/chryswoods
to get what is currently the absolute latest version of the code.
However, while you can download this code whenever you wish, I would not currently advise it. Compilation of Sire is not yet straight-forward or user-friendly, and the code is still buggy and in need of work. Sire is not ready for use, and at this stage, it is only worth downloading this code if you wish to study it, or are planning to make a contribution to it.