You can obtain anonymous read only access to the CVS repository as follows:

cvs -d :pserver:anonymous@cvs.spl.harvard.edu:/projects/cvs/slicer login

Respond with the password bwhspl.

Follow this command by checking out the Slicer 2 code and documentation:

cvs -d :pserver:anonymous@cvs.spl.harvard.edu:/projects/cvs/slicer co slicer2

To request write access to the repository, please email slicer@ai.mit.edu.
Please be aware, that according to the software license that applies to the Slicer code, that the copyright of code contributed to the cvs repository is signed over to the copyright holder of Slicer. Please see the Software License Agreement for further details, as it also applies to the source code, not just the distributed versions.
Access to the web interface to CVS is available via the following URL: http://splweb.bwh.harvard.edu:8000/cgi-bin/develop/viewcvs.cgi/slicer2/?cvsroot=slicer