Typically, service providers provide a site collection to a customer, and the customer administrator is given the role of "site collection administrator." For more information about this subject, see Choose administrators and owners for the administration hierarchy.
If your customers do not want to modify their site through the standard Windows SharePoint Services site administration and configuration capabilities, they may want to use Microsoft Office SharePoint Designer 2007. See Microsoft Office SharePoint Designer 2007 Home Page for more information about how to download this tool.