Balsamic vinegar (especially aged vinegar) has a great flavor that intesifies and sweetens when you reduce it. Bring a small pan of balsamic vinegar to a boil and then simmer until it thickens. When it coats the back of a spoon, it's done. It makes a great salad dressing or veggie dip and is extremely low calorie!
If it's too tart or strong for you, you can dissolve a bit of sugar in the vinegar as it's heating; however, I prefer just straight vinegar.