The advanced user module allows the filtering of users based on the user.module fields and optionally the profile.module fields. The fields available for filtering can be configured using the module settings. Eg. Site admin may search through 1000s of users to display all users who have not accessed their account.

Once the group of users with selected common denominators are filtered, the module can be used to perform functions that email, block, unblock, add or remove roles, or delete. Another nice feature is the ability to notify administrators or other roles, by email, of user account creation or profile changes.

http://drupal.org/project/advuser