Unless you're looking for something specific, repackaged salad dressing is an unnecessary expense. With a few ingredients you probably have anyway and an understanding of emulsification, you can go from spending money on salad dressing to making your own healthier, tastier dressing at home.
Over at Gilt Taste, you can find a detailed explanation of the process in plain English, along with some suggestions for making better vinaigrette at home. The process is a little more intricate than oil, vinegar and an emulsification agent, but not by much:
Oil and vinegar, as everyone knows, don't mix. You can force them to do so by shaking or mixing them violently, but they'll eventually part ways. But you can compel them to make friends through by introducing an intermediary. Garlic and mustard make very good ambassadors, but there are other foods which work equally well: blue cheese, catsup, a touch of miso, and of course egg yolk, which is perhaps too good an emulsifier for vinaigrette-it will make your dressing seem more solid than liquid.
The piece goes on to help you pick the best oils and vinegars for your at-home salad dressing. Don't be frightened by egg yolk either -- I keep a squeeze bottle of homemade salad dressing in the fridge (even though Gilt suggests making it fresh every time) that I make with mustard, garlic, rice wine vinegar and olive oil (and just enough salt and pepper, of course) that goes well on any salad.
The key is getting the proportions right, learning your own tastes and experimenting. They have some suggestions at the link below. Once you have a good combination down, you'll never waste money on a bottle of dressing again.
How to Make a Better Vinaigrette [Gilt Taste]