Downtown in many cities consists of buildings with innumerate glass windows, all of which slowly collect dust and must be regularly cleaned. Generally, this is done by a suspended crew, which is attached to the roof via a long rope. Various companies to do this exist in various cities. They charge a fee for this, a fee that I do not know.
If one wanted to not pay this fee, I can design a building which would wash its own windows, automatically. Each window will contain, in its sill, a mechanical squeegee, and a pipe to a supply of soapy water. Periodically, the soapy water would run over the squeegee, and the squeegee would slide to the bottom of the window with an elaborate system of gears in the cracked. It would then be worked up, and rinsed. The squeegee would be repositioned within the sill to have the rubber side face the window, and a jet of fresh water would run behind it. The rubber end would be brought down to the bottom of the window, and then back to the top. The window is now clean.
Alternatively, the soap and water solution can be replaced with ammonia, which does an excellent job at cleaning windows.
I predict few buildings will incorporate this system, which is a tad impractical.