While there is a wide variety of different products that are designated as "window cleaners", it is best to use soapy water. It may not seem as though soapy water would have the same cleaning qualities, but it will be more than sufficient. Not only is soapy water a great cleaning agent, it is also safe to use. Of course, when you are cleaning the actual glass panes themselves, a glass cleaner can be used, but be sure to check that it is water-based.