Some sites like to run as an outsourced service. For example, a payment service, a support ticket system, or an event registration is not something every site wants to run. As a solution, they want to pay some other specialized site for this service.

Let us say that Widgets Inc wants a support ticketing system, and contracts Support Corp to do this service over the web for its clients. Support Corp's site wants users who come from Widgets Inc's site to have the same branding and look and feel as Widgets Inc's site.

At the same time, Support Corp has been contracted by other companies like ACME, Sprockets, ...etc. Each company wants Support Corp's web site to look like its own web site for its own clients.

To do this in Drupal, I have written a new module, called Referer Theme. It relies on changing the theme based on the host name that is contained in the HTTP_REFERER. Each hostname is assigned a different theme.

Moreover, there is a timeout value after which the theme reverts to the default Support Corp. 

This way, Support Corp can have the same web site server different corporations with unique branding and look and feel for each of them. 

Update 2006-01-23: The referer theme module is now available for download at Drupal.org.