For instance in our family, baby items - from the big (cribs, changetables) to the small (clothing, bottles, toys) - are constantly passed around from brother to sister to cousin and back again. It would be hard to quantify those types of savings, but I'm sure it's substantial.
Another way we save money among family (and friends) is that we each buy a certain type of magazine - say OK! Canada or US Weekly (or say MoneySense, whatev!), and then we swap when we're done, eliminating the need for multiple subscriptions. Movies and books get passed around like this, as well.
What are some ways your own circle of family and friends have saved, or helped each other save, money?