I love getting a bottle of dressing for free with a coupon, but nothing beats a freshly mixed vinaigrette. My favorite vinaigrette recipe comes from 1,000 Lowfat Recipes, by Terry Blonder Golson. Check it out on Amazon, it's one of my favorite cookbooks.
Balsamic Vinaigrette
1/4 c. Extra Virgin Olive Oil
3T Balsamic Vinegar
1T Lemon Juice
1/8 tsp. kosher salt (other salts will work if needed)
1/8 tsp. freshly ground pepper (again you could substitute)
Whisk together all the ingredients. This will store in the refrigerator for quite awhile. I keep mine in one of those squeeze bottles, like they put on the table in restaurants.
Do you have some other kind of vinegar or oil on hand? If you want to take a swing at making your own vinaigrette, try this great article