If you do, how much do you leave?
Yes, I tip. There's usually a little envelope, signed by the cleaner, for that purpose. I usually tip a dollar a day. (I'm using the more generic "cleaner" because I've frequently had male room cleaners and "maid" just doesn't feel right.)
You may want to tip on a daily basis if you stay for an extended stay. Your room may not be cleaned by the same housekeeper every day.
That's why I tip every day.
I once didn't remember to leave a tip till I was on the way to the elevators to check out and the cleaner was already in the room. The guys in the hall were shocked when I walked up to the guy cleaning my room, apologized, and handed him a couple bucks.
Tip: $2. Shocked cleaning crew's expression: Priceless.