For our foreign friends visiting the U.S., one thing to keep in mind is that the pay scale in restaurants here takes tips into account. Waiters, waitresses, bartenders, etc. are paid under the assumption that they will receive 15-20% tips from customers. Therefore, they depend on tips to make a living. In many other countries, by contrast, the pay scales tend to be a bit higher since tips are much more variable, thus your waiter or waitress won't starve if you don't leave much of a tip. That may help put things in perspective.
As for exact numbers, in restaurants 15% is sort of the minimum under normal circumstances--I think in New York, at least, most people give a bit more. Exactly 15% is still perfectly acceptable, but you should only give less than that if the service is particularly poor and you want to convey your dissatisfaction to the staff. As others have suggested, I usually double the sales tax (8.625%) and then round up a bit, which is quite easy (even after a few drinks...) and works out to 17-18%, which is generally about right. Note, in case it wasn't clear, that you should calculate the tip based on the pre-tax total!
Anyway, back to the original question: for bellhops, if they carry your bags, $1-2 per bag is fine. For taxis and airport shuttles, approximately 15%, rounded up to the nearest dollar. Again, as others have pointed out, feel free to give more or less depending on politeness, driving skills, etc. Occasionally in NY I get a cab driver whose heavy-footedness on the gas and/or brakes leaves me fearing for my life and/or nauseous. In those cases I definitely give less of a tip! On the other hand, sometimes I get a great cab driver who knows shortcuts to avoid traffic, or just a nice guy who is chatting about his family back in Bangladesh, and in those cases I'll give a little more.
Common sense is not so common. -Voltaire