I generally DO leave tips for housekeeping, at least in countries (such as the US) where tipping is the custom. Usually it's only a buck or two a day, depending on the size of room and quality of service. (Sadly, sometimes I forget. )

I put the money in one of the hotel-supplied envelopes, seal it, and write "Housekeeping" on it so there's no mistake about my intentions. If the hotel is a cheap one that doesn't supply envelopes, I take a piece of notepaper, write "Housekeeping" on it, and wrap it around the money. Whether in an envelope or a piece of paper, I leave it somewhere obvious, such as on the nightstand or desk.

Oh, and I DON'T leave cash or other valuables just lying about in a hotel room while I'm not there.