The following plugins come bundled with the core module and can be enabled or disabled according to what functionality the site own would like

User Relationship Mailer
Will (conditionally) send email notifications regarding relationship creation/removal/approval/disapproval/cancellation.
User Relationship Defaults
Creates default relationships to any user joining the site (think Tom on MySpace).
User Relationship Implications
Allows admins to specify implied relationships (Manager implies Coworker) that are automatically created when this implying relationship is created and removed accordingly.
User Relationship Blocks
Provides several different blocks.
User Relationship Invites
Requires the Invite module and allows users to specify a relationship to a user that they invite to join the site.
User Relationship Views
Provides integration with the views module providing filters, arguments, and fields.
User Relationship Privatemsg
Integration with the privatemsg module showing your relationships in the quick select list.
User Relationship Migrate (Drupal 6 only)
Allows admins to migrate Buddy List 2 to User Relationships.
User Relationship Elaborations
Lets users send a message with their relationship request to another user.
User Relationship Node Access
Lets users open up access to their content based on relationships
User Relationship Services
Exposes UR functions to Services module
User Relationship Rules
Provides rules integration for relationships
User Relationships Panels Visibility
Provides pane visibility rules based on relationships.

Most of the following linked pages have not been updated since Drupal 5. Feel free to edit and move to the latest version to help out our current users.