One of my good friends had a whole host of imaginary friends growing up. Her mum vividly recalls how she had to set an extra place at the table for a certain one, another one needed to be tucked in at night, and still another one accompanied her to school.
My mom is a preschool teacher and says that one of the little boys she teaches has an imaginary friend who comes to school with him. One day he was sitting on the swings and talking to his "friend", and my mom swears the empty swing next to him started to move! Probably just the wind, but it freaked her and the other teacher out!
Last but not least, I know a Christian counsellor who says that imaginary friends are actually guardian angels, and that children who have them can actually see and talk to their guardian angels as they're "close to heaven", while we grown-ups are "too far from heaven" to be able to communicate with ours. I like that take on things.