I call all my best friend's aunts and uncles "auntie so-and-so" and "uncle so-and-so". I'm pretty much a part of their family now so it's expected
A family friend of mine used to be called "uncle" when we were much younger...now that we're all older the age difference between us (12 years) seems much less, and so he no longer is eligible for uncle status, hehe.
When I have children, my best friend will definitely be an "auntie" to my kids. No doubt about it. I firmly believe that a child should have a lot of adults they feel they can turn to--blood shouldn't matter in this kind of relationship.