From the time I was old enough to go out of the house by myself, it was drilled into my head that you always carry id and money for a phone call. I've got the RoadID bracelet and wear it any time I go cycling, kayaking or hiking. If you go to the RoadID website (oxysback provided the link above), they list a lot of valid reasons for wearing/carrying id. For instance, if you have certain drug allergies or medical conditions, it could be life-saving for an EMS technician to know this before treating you.

I've actually been thinking of getting one of the new interactive versions so that I can include a note that I have two cats at home and that whoever finds me should contact my pet sitter. It's one thing if my family doesn't know for days that I'm in a coma in the hospital, but it really bothers me to think that no one would be feeding my kittehs.