It depends on the country and the service provided.

If I'm in France, Germany, Belgium then I don't tip since the housekeepers are paid a living wage just as the waiter/waitress in a restaurant is paid a living wage.

If I'm in Eastern Europe and receive good service and the room is clean then I tip a few $'s/day. If I'm in Asia then I generally don't tip as I honestly don't know which countries tipping is accepted and which countries it is frowned upon.

In the US I will tip if I've received good service and the room is clean, but truthfully speaking that is few and far between. US and Western European hotels are the worst when it comes to pounding on doors at 10 minutes to 8am saying "housekeeping" - why can't they just get a printout of guest that have checked out???? Pound on my door and guarantee no tip.

At any hotel that charges a service fee I don't tip. I personally have only stayed at one hotel ever that I felt the service fee was in proportion to the services provided.