Role Referral allows the assignment of roles to anonymous users who enter the site at a specific URL ([drupalsite]/referred) from a specified referring URL.
This module was developed to address a specific feature request by a client. Its primary intended use is in conjunction with an access control module (such as Taxonomy Access Control Lite, for instance), to make certain content hidden to the general public but immediately visible to anonymous visitors from certain pages (such as from the member-only section of the website of a partner organization) without them needing to first go through the hassle of creating an account.
More information and download available from the main Drupal site: Official Role Referral Drupal project page.
It's currently only available for Drupal 6.x.