This one is super simple and inexpensive and in my opinion works better then traditional window cleaners.
It also kills mold, mildew, bacteria and is NON-TOXIC.
Here is what you will need:
White Distilled Vinegar
Simply fill the spray bottle 1/2 full with Vinegar and then fill it the rest of the way with water.
Spray on your windows and clean as usual.
If you do not like the smell of vinegar, I have heard rubbing alcohol and water work to, but I have not tried that recipe.
That is it. Nothing more, nothing less. 1 Gallon of White Distilled vinegar can keep you cleaning windows for less then a penny every time.