Conde Nast Traveller magazine has a webpage on tipping rules by country which covers different situations and establishments (hotels, restaurants, taxi drivers) for 50 countries. It can be a helpful reference on the matter. I personally think the article is aimed at travellers coming from countries where tipping is customary, if not mandatory, and is therefore geared towards overtipping. Just as an example, I've stayed in countless hotels in Italy and have never tipped housekeeping, as opposed to the suggested 1–2 euros per night. However I have tipped taxi drivers several times, especially when I've had them wait after calling or when I've travelled with a lot of luggage. Nevertheless one could use the linked article as guidelines indicating the upper bound of tipping amounts, i.e. the maximum one should consider tipping per country.
Many more such guidelines can be found on the web. Four example Fodor also has an article on tipping around the world, although I find it less comprehensive.
All in all when it comes to housekeeping the general consensus is that tipping once per day will get you better service. We've also discussed this on Travel SE already. If you're in countries where such tips aren't expected, I would leave a note clearly stating that the cash you leave is for housekeeping to take, to avoid unpleasant situations.