The answer to 'natural window cleaner' is to use a simple, homemade solution of white vinegar and water. Mix equal parts vinegar and water in a spray bottle, then use a microfiber cloth or squeegee to wipe down your windows. This natural cleaner effectively cuts through grime and leaves windows streak-free.
For extra stubborn spots, you can add a few drops of liquid dish soap to the vinegar-water mix. Be sure to rinse thoroughly afterward. Avoid using this solution on tinted windows, as the vinegar may damage the tint.