Cycle helmets: wear 'em
It is a frequently angry, everlasting debate among cyclists: helmets or no helmets?
I'm in one of the extreme groups: I won't go out without one.
Partly that's a function of coming from Australia, where they are compulsory, and partly it is because I think for the small amount of inconvenience involved you could save your brains. (And I don't go for the argument that it makes either you or motorists more careless - I know I'm still extremely vulnerable, and I doubt most drivers are paying enough attention to notice.)
This scary tale posted by a doctor backs me up, and shows that it is not just the motorists you have to worry about.