Tipping is definitely a cultural thing. Here in the US, people who are tipped are paid less by their employers (in fact, they have a lower legal minimum wage). Therefore, it has become more of a expectation than a way to say "you've done an exceptional job." A 15% tip is considered standard for a waiter/waitress in a sit-down restaurant with table service. Cabbies are often tipped at around 15% as well. One often tips a bartender $1 or $2 per drink or 15% for the bill.
Personally, at a restaurant I usually aim for around 20%. I will do 15% for nearly adequate service. Usually it takes activity hostility or gross incompetence before I'll tip lower than that.
Tipping is an odd thing, though. It's one of those cases were extra is expected. I suppose on the plus side it's a way to give direct feedback about the worth of someone's work.
EDIT: I would totally suck as a waiter, so I am extra-appreciative of those who do a good job.