Was commuting to work on a big black MTB w/toe cages, and a front suspension, heading SE on Milwaukee. The light changed before I got to the intersection, so I stood up and started pumping, just getting up to speed when someone opened their door right into me. It was an older PR lady, who couldn't stop apologizing afterwards. I stood up, closed my eyes, and tried to relax. I went over the door, the bike followed me, and tomahawked into the car parked in front of the one in front of me. I hit the ground head-first (cracked helmet in 2), followed by my shoulder (goodbye, skin), and then finally splatted on my back. Luckily I'm super solid, so I managed not to break anything. Helmet saved my life. Guy who was riding a bike behind me thought I must be dead. When I got up and started to get mad at the situation, he handed me his card, and slowly pedaled away, incredulously.
I may have told this in another thread- a friend rode his bike to Akido class. He left his helmet at class and started to ride home. He realized his error and went back to get his helmet. He resumed his trip home. A large branch fell off of a tree landing on his helmet. He rode home with no injury. Sure, if he did not return for the helmet that particular branch would not have fallen on him. Other than the cosmic time/space issues involved with the when and the where my friend's life was saved because he had his helmet on his head when the branch fell upon him.
Most importantly, when your helmet hits the pavement (with your head in it) it makes a super cool sound! [true story, from a hit in 2009]
I didn't wear a helmet for years, after always being forced into one as a child. Had my first bike + car crash in college without one, and decided it wasn't worth the risk now that I knew what the side of a van felt like. My parents and I had invested a lot for me to be in college, so I figured it was the respectful thing to do to protect my brain. But this time the helmet had to be awesome, as different as possible from the stupid ones from childhood. For me that meant a shit ton of gold stickers, but whatever – he's got to want it.