Tipping is a strange custom, I agree.
Nevertheless it is inbred in the US system to the point that
employers can pay people LESS THAN MINIMUM WAGE if they are in jobs that are routinely tipped. (see
http://www.dol.gov/esa/whd/state/tipped.htm). This includes Hotel Housekeepers.
The problem with this is that depending on where you work, you may get to pool your tips (which can help get you up to minimum wage), or you have to self-report, or the employer can 'assume' you are getting the tips listed in the tables (they are not supposed to assume this, but indeed, many do).
So waiters and hotel workers do not get minimum wage prior to their tips - their tips are calculated in. If they are not tipped, then, the employer is supposed to make up the difference to minimum wage.
Tipped positions, as has been stated, include waitresses in restaurants where they bring the food and drink (15 to 20%), people who bring drinks only at a buffet ( 2 or 3 dollars), bellhops who carry your luggage ( 1 or 2 dollars per bag), housekeeping (1 or more dollars a night, depending on how messy you are, etc., we usually pay 2 to 3 per night, Paid nightly, since staff changes), taxi drivers, tour bus drivers - I usually look on the internet for common tipping amounts for these since we rarely have used these services. The only other pitfall you might encounter in terms of etiquette is at a high end restaurant, where you tip the waiter and the somalier separately - and sometimes the maitre de as well! Bartenders are also tipped a dollar or two per drink.
Of course, you DO NOT have to tip, every service person is used to being stiffed for whatever reason, and you may get such terrible or non-existent service that you want to send a message, but the OP asked about the etiquette of the thing.
