We tip all those regularly, including taxi drivers. Hairdressers, doormen, valet parkers, the guy at the hotel who gets you a taxi, delivery people, full-service gas station attendants (which really exist only in New Jersey), exotic dancers (well, I haven't had the chance ), the supermarket bagger if they carry your bags out for you. It's expected depending on the quality of the service, so leaving no tip or an insultingly low one - I left a penny once in a bad diner - is a big deal.

Workers who are regularly tipped like waitstaff make a significantly lower minimum wage by federal law with some variation between states. (Can you say big corporations passing labor costs on to the consumer?) Which is why I refuse to put money in a tip jar - a Starbucks barista is making a higher non-tipped wage already.