Passive cooling

Got a window that lets the hot sun in during the summer? Plant annual vines to shade it. It's still early enough in most parts to plant morning glories, black-eyed Susan vines, cup and saucer vines or a double duty vine like (edible) scarlet runner bean. You'll need to offer some support - a light wood or metal system of some kind, but that needn't be expensive if you make your own or creatively use old grills, stakes, etc. Plant the vine so that it will shade the window during the hottest part of the day and take into consideration the angle of the sunshine later in the year.

Cheap window cleaning

Don't use cleaners, just water. Unless your windows have bird stuff or grease on them, plain water will do the job just as well as "cleaners." And if you just have to use a cleaner, for heaven's sake don't buy it. Use a little vinegar or a drop of dish detergent in water. Why pay for something you don't need?

Fool your neighbors

This might not be officially "frugal," but it addresses a problem frugalites sometimes have: finding ways to bypass the questions and strange looks.

Use a little vinegar or ammonia in water to wash windows - that's commonly known - but if you want to fool your neighbors (spouse, mother-in-law, etc), put a drop of blue food coloring in it. They'll think you're using a commercial window cleaner. If you can come up with an old spray bottle with a brand name label on it, so much the better, but maybe you can fake it somehow. All in fun, of course.