Use low-fat mayonnaise. It is fine on sandwiches and mixed with strong tasting foods like tuna. This really cuts the fat content of the mayonnaise and makes an excellent substitute for high fat mayonnaise in potato salad and other mayonnaise based salads. The only problem with them is that they do not hold well. I make just enough for one salad by using part of the dry ingredient from the packet and an approximate amount of the other ingredients.
Balsamic vinegar is a great salad dressing, all by itself.
It is also great sprinkled on ripe tomatoes with shreds of basil on top. Non-fat sour cream breaks down in cooking and essentially becomes water. Non-fat sour cream works well in dips if you mix it with equal parts drained yogurt. The yogurt gives the sour cream more texture. Just be sure that you purchase yogurt made without gelatin.