Ammonia-based cleaners & a squeegee.
I imagine that commercial window washers use different products so it may be hard to pinpoint one product that they all use. A good window washing solution is 50:50 water and vinegar with just a small drop of dish soap. Commercial window washers are often seen using squeegee's but I always get streaks when I try to use a squeegee--which is why I'm not a commercial window washer! I use a sponge to apply this and then use wadded up newspapers to dry the windows. You will have nice, shiny, clean windows that stay clean for a while.
My aunt saw TSP being used in water by her window cleaner.