I haven't found many fat free dressings that taste remotely edible to me (and those that do always seem to be ridiculously overpriced), so I just make my own.
I've perfected the splash and dash method, but initially I started with online recipes, but found that I can use little or no oil, if I use rice wine vinegar or citrus juices that are milder than typical vinegars.
My favorite is just rice wine vinegar (or the vinegar from a jar of pickled yellow pepper rings), and a pinch (each) of splenda, garlic powder, and Lowry's seasoning salt.
I also love the oriental salad dressings you can find in asian grocery stores or the ethnic sections of the grocery store. I can't remember a particular brand, but they're named after the color "green salad dressing," and "red salad dressing." The green one is awesome. It's a puree of ginger, cilantro, and garlic (makes a great marinade or potato/pasta topping too).
I also love the red one, but I love hot peppers. It's pretty spicy because it's chili based. Hubby likes the green one (it reminds us both of house dressings in japanese steakhouses), but he's a wuss when it comes to chili pepper heat so the red one is too hot for him. (Now in the case of horseradish, mustard, or black pepper he's the fan and I'm the wimp).
I also use bottled Nuoc Cham (thai dipping sauce) as a salad dressing. Really yummy, and very low fat and low calorie. I now usually make my own (it's really easy and there are tons of recipes online. I just replace the sugar with Splenda).