If there's one thing on-demand car startup Uber likes more than ferrying people around, it's trying to grab people's attention with kooky promo stunts. Uber for barbecue? Uber wedding packages? Pairing riders with attractive lady drivers? Been there, done that (for better or worse). Every once in a while though, Uber cooks up something genuinely useful and today is one of those days: if you live in Boston, Washington DC or New York City, you can order an on-demand flu shot for you and up to nine of your friends until 3PM Eastern.