If you're a runner, biker, or otherwise might get incapacitated away from your wallet, you should carry an ID. I recently got one from these guys and like it. They have a couple kinds, but I got the "Road ID Slim" which is a small ID on a standard silicone bracelet. They are pretty cheap: $15 and you can put any text you want on it. (I put my name, contact numbers, "no allergies", etc.)
If you want to get one of these, you can use this coupon code and get a buck off: ThanksShannon13853851. (I don't get kickbacks or anything, I just like the ID I got.)
Road ID provides premier line of identification tags that includes runner id, jogger id, cyclist id, medical id, bicycling id, emergency medical id bracelet and more.