You can use the code from these repositories to experiment, test, build, create patches, issue pull requests, etc.
hudson/org.eclipse.hudson.core.git
hudson/org.eclipse.hudson.maven-hpi-plugin.git
hudson/org.eclipse.hudson.remoting.git
hudson/org.eclipse.hudson.stapler.git
hudson/org.eclipse.hudson.test.harness.git
hudson/org.eclipse.hudson.test.ui.git
www.eclipse.org/hudson.git