I have always tipped housekeeping at hotels. These people most often work for minimum wage and a tip is a nice way of saying thank you for doing a job few of us would choose to do. We leave a mess and they clean it up. Sure, they are paid for it, but c'mon...
At Disney, we print Mousekeeping envelopes to leave daily. Our standard is $2/person, but if we left a mess, I may drop in some extra $. We also usually leave a note inside -- just something like "Thanks for taking care of us" or "Thanks for being part of the magic."
Sometimes, the mousekeepers have left extra amenities for us or extra towels. A few times they left us a nice note thanking us for the tip. A couple of times, the mousekeeper left us a towel animal! It was really nice.