Ranch dressing is pretty good, and it's only 1/2 carb per tbl (70 cals).  I like
to use Marie's guacamole (.15 carbs/45 cals) most of the time.  Sometimes, if I'm
feeling decadent, I'll use plain old mayo (.4 carbs/99 cals).  Google this group
for salad dressing and you'll find a LOT of recipes and ideas.  I usually eat a
pretty big salad (as a meal in and of itself) and find that 2 tbl of whatever
dressing I choose is plenty.  

Check the label for sugar and its forms (-ose, corn syrup, etc.).  I used
Kraft Ranch dressing and Good Seasons Italian.  Yes, I know that they had
traces of sugar, but it wasn't enough to stall me or spike my blood sugar.
Just generally check for dressings that have low carb counts (1 or less per
tablespoon).  I tried some specially formulated low carb smoked tomato
dressing and didn't care for it (too sour!).  Don't buy "low carb" specialty
salad dressings, unless you can taste it before you buy.

Linda

i use bottled salad dressing most of the time.
look at the ingredients and NOTICE that the LOWER the fat, the
HIGHER the carbs.
forget all the OLD rules and forget counting FAT grams, its tough,
but a wonderful habit to get into!

There are a number of dressings in stores that you can use, including
Ranch, Italian (but not Creamy Italian), Blue cheese, some kinds of
vinaigrettes, and Green Goddess (which, IMO, is like creamy Italian
without the sugar).  Check your labels for ones that are only 1g or 2g
carb per serving, and count the carbs.

I also use Good Seasons Zesty Italian packets, made up with a cold-pressed
oil and cider vinegar.  Granted the mix contains some sugar, but it's
only 1g per serving.