Decoupled User Authentication
Project site: https://www.drupal.org/project/decoupled_auth
This module aims to provide a simple API for storing information about users who visit your site whether they register or not. By extending the base user entity the module allows you to store users that do not have a name and password.
Drupal has thousands of useful contributed modules and many of these work with users really well (e.g. Simplenews, OG and Profile). These modules would be even more effective if they could be used with users that are not registered.
For example, storing profiles for unregistered users opens up whole new possibilities for using Drupal as a CRM framework.
Drupal Commerce could also benefit from this by allowing users to give an email address but not register before going through the checkout process.