CAcert Source Code
The project has several parts.
Main web application in PHP: http://www.cacert.org/src-lic.php (See Software/DevelopmentWorkflow if you would like version control support.)
Signing Server Service: http://svn.cacert.org/CAcert/CommModule/
Application Database: http://www.cacert.org/sqldump.php
See InstallatingCAcertSource for install instructions.
See DeveloperNotes for notes and comments
See Testing for notes and comments
Software Webdb Documentation: Full Webdb Documentation