The source code of Aegir is located on Drupal.org, as with the issue queues and release nodes.
Aegir is also made up of several separate components designed to work together, that are grouped in two main git repositories: hostmaster and provision. This page details those locations.
There are the URLs you can use to pull the repositories:
git clone http://git.drupal.org/project/provision.git git clone http://git.drupal.org/project/hostmaster.git
Developers with push access to the repositories should pull via SSH.
git clone ssh://[username]@git.drupal.org/project/provision.git git clone ssh://[username]@git.drupal.org/project/hostmaster.git
There is a web interface for your perusal at:
The git repositories were hosted for a while on Koumbit's servers, but were migrated back to Drupal.org during the great git.drupal.org migration. Do not use git.aegirproject.org as it doesn't have the most recent commits and may be shutdown at any time.