A way to link a person to another contact - say husband and wife with different names or link a contact to a company so you can look at as the person or listed under the company.
Stephen Moore commented
I would like to be able to view a contact and see a list of other contacts that I linked to them for referral/lead management.
In Deals, you can associate a contact with a deal. Why not have an associated contact just within the contact screen? Even in the deal screen it should be able to let you describe the reason they are associated.