I just bought Hellmanns fat free vinaigrette dressing from Tesco. Can't comment on how it tastes yet as I haven't tried it. Frylight also do a caesar salad spray dressing which is free but it tastes very strongly of garlic and wouldn't be everyone's cup of tea.
10 tbs balsamic vinegar, 1 tsp. honey, 1 tsp. mustard whichever mustard is your preferred one, I use Coleman's powered English mustard, makes a lovely dressing. Put the lot in a screw top bottle and shake. Kept in the fridge it lasts for ages. I think it is about 1 and a half syns and as this makes up enough for at least 5 or 6 meals the syn value becomes negligible.