By a soccer ball. We were doing that 'kick the ball up into the air as high as you can' in grade... I dunno, 5? Anyway, I got distracted by someone, and ended up hearing 'HEADS UP', just in time to look up and take the ball square off the face.
Thinking about it now, it must have looked totally hilarious. I imagine my body going all limp, collapsing on the field. It was kind of strange actually - I'm almost certain I lost most of my memory for maybe a few minutes before that. Specific things, like walking out onto the field at recess before we started kicking the ball around, for instance - I had no recollection of ever doing that. Anyway, I woke up a few minutes later being carried by a teacher into the school. Spent the next little while in the nurse's station until my folks came and picked me up.
I didn't break my nose or anything - people who saw the whole thing said the ball pretty much nailed me square in the forehead. Just a bit of a headache later...
The practice of kicking a soccer ball straight up into the air was banned after that. All because of my ADD.