This is the BEST fat-free homemade dressing I have ever had. It does have a bit of a Dijon tang to it though, so if you don't like the flavor of Dijon you might want to replace it with regular mustard or another flavor.
1/2 cup non-fat mayonnaise
2 tbsp grated nonfat Swiss cheese (I just use regular Kraft Parmesan in the green can - works out just fine)
1 tbsp lemon juice (I actually do squeeze a REAL lemon, are you impressed?...but I'm sure ReaLemon would work fine too.)
1 tbsp red wine vinegar
2 tsp Dijon mustard
1/8 tsp black pepper
Put in a cruet and shake, shake, shake. (After it sits in the fridge for a while, I have to add a little water before using because it gets a little thick for my taste. I like liquidy dressings.)