I think there are a few bottled/jarred red salsa products that are fairly low carb. It's been awhile since I was on standard Atkins, but I seem to recall that La Victoria Salsa Suprema in Medium is about 2 total carbs per two tablespoon serving - a pretty good 'carb bargain' given the nice flavour punch you'd get from the salsa.
There are *lots* of low-carb salad dressings on the market - blue cheese, ranch, various vinaigrettes. I used to search for the ones that were 1 gram or less of total carbs per one tablespoon serving. The ones in the jars in the produce section were pretty good.
The trouble is - because these are so thick and creamy, they don't necessarily coat your salad well, and end up in this clump on top of your salad/meat/vegetables. Then...you end up scooping up way more of it...adding in more carbs in the process.
A good solution? Make up your salad, and then plunk it into a mixing bowl, and measure out the seemingly measley 1 tablespoon 'serving' of dressing on top - and then mix it all up with a wooden spoon or spatula, as if you were making a caesar salad. Voila! You've stuck to the portion size, and you get all the flavour from the small-ish serving without going over on your intended carb count.
Using this method of 'tossing' with the dressing, I think you'll find that just one tablespoon of the dressing is plenty.
editing to add: check to see if there's Santa Barbara 'fresh' salsa in your local grocery store - these come in little tubs and are sold refrigerated - very tasty. Low carb, and I like the pico de gallo and the green tomatillo salsa.