Many people have already added projects to drupal.org that are sub-themes of Zen. Documentation on contributing a theme to drupal.org can be a bit tricky, but if you find you are having problems figuring out the process, there are lots of people who are willing to help in IRC. You can also open an issue in the Zen issue queue to ask for help. :-)

The following is an incomplete list of Zen sub-themes that have already been contributed to drupal.org:

Sites implementing Zen

You can view a gallery of websites implemented with Zen at the Zen Theme Garden.