Affinity could be defined as "a close connection marked by community of interests or similarity in nature or character" (WordNet). I've always complained that Gmail doesn't show statistics about your messages, your contacts, but I've just discovered a cool way to find out something more about your Gmail contacts.
If you open this XML file (you should be logged in), you'll discover a list of Gmail contacts sorted by affinity and a value for each contact. The value could be calculated by taking into account how often you write to each other or maybe even deeper knowledge of personal interests.