I travel a lot for the last few years (last year 220+ nights in hotel) and tend to tip:
* waiters
* bellboy if they carry my bags (but I do not use bellboy for only carry-on)
* maid (on average about $1 a night but sometimes $5 on a 4 night stay)
* hairdressers
* taxi - I tend to round it up but usually get reimbursed for it
For hotel conceirge and front desk staff, I tend to do Belgium chocolates at Xmas or when I leave if they had been good to me.

I am now in a place where tipping is not expected normally but still tips occassionally when the service has been very good even though it is not the norm.

But I resent being required to tip and those people who still expect a 15+% tip even when the service has been lousy. I had a friend in the UK that actually called the manager at a restaurant in London to the table and got him to remove the service charge from the bill since the service has been terrible.

Just my 2 cents