This is a Drupal 5 multi-site setup of 7 sites, that share a single base theme, with a sub-theme for each specific site. When users sign up, they are automatically signed up to all sites. There are a few custom modules that handle the category, trade name and company lists, and the product compare option. Most of the custom blocks are stored in a custom module, which makes updating all the sites with the same code much simpler.