The Zope Toolkit code is maintained via Subversion in the official Zope repository at svn://svn.zope.org/repos/main.
The official code browser is reachable at http://svn.zope.org.
An additional browser using Trac is available at http://zope3.pov.lt/trac/.
There is a wiki which contains the guidelines for how to work with the repository including an introduction for new committers: http://www.zope.org/DevHome/Subversion/FrontPage