Most have laundry service. I've used Wynn, Caesars and Bellagio laundry services.
They pick your laundry up in the morning, have it back to your room in the afternoon. Pants $7-8, shirts about the same or a little more, depending. They even do undies.
If you are speaking of hotels having coin-op washers/dryers available for guests to use, I don't know of any.
Some of the extended stay places have them, like Hilton Vacations?, or so I've read.