Valet: I choose to have this extra service. At casinos it's free, so I tip for it.
Cocktail Waitress: Same thing.
Bell Hop: Same thing.
Restaurant Server: This one's different. These folks are doing what they're already paid to do, but they interact with you multiple times in order that they make sure you got what you want the way you want it, and also to make your dining experience as enjoyable and pleasant as humanly possible. I tip gererously for this, and there's of course times the experience was bad and no tip was left.
Housekeeping: I don't tip the maintenance people who make sure my room has properly working stuff in it; I don't tip the security guards who make sure the hotel's guests are safe; & I don't tip the people who make sure our rooms are clean. It is all included in the price of the room, comped or not. I believe the reason people tip housekeeping and not the others I mentioned is because there's the possibility you will bump into these people during your stay. And since housekeeping has the slimy practice of putting tip envelopes in rooms, the intimidation factor purposely gets cranked up. However, if I request a special service like an extra pillow or more towels, I always tip the person who brings them.