Your phone. Via Sciencemag.org:

Just by analyzing the calling patterns, the researchers could accurately label two people as friends or nonfriends more than 95% of the time. But the results, published online today in theProceedings of the National Academy of Sciences, show that the mobile phone data were better at predicting friendship than the subjects themselves. Thirty-two pairs of subjects switched from calling each other acquaintances to friends in the traditionally gathered survey data. These are most likely new relationships that formed during the course of the study, say the researchers, and they left a clear signal in the mobile phone data. Friends call each other far more often than acquaintances do when they are off-campus and during weekends. The pattern is so distinct that the researchers spotted budding friendships in the phone data months before the people themselves called themselves friends.