Tomatos are a healthy staple of any diet, most definitely including low carb diets. Tomatos are full of great vitamins and are low in excess sugars.
A half cup of tomato only contains about 3.2g of carbs. Compare that with the 14g+ found in potatoes!! Plus you get way more nutrition out of your tomato.
What's fascinating is that only a short while ago, mankind thought that tomatos were poisonous! That's because the plant itself (the green part) is not healthy to ingest. However the bright red objects we eat are quite good for us.
What's even more fascinating is that a tomato is actually a fruit. It has all the botanical properties of a fruit. The only reason we call it a vegetable is because we tend to eat it as a food, rather than a snack. Interesting!
Some common types of tomato include:
Beefsteak - the large ones that are used, in slices, on burgers
Cherry - the teeny ones that are like marbles, used in salads
Roma - medium sized oblong ones, perfect for salsa and sauce
Some people try to figure out the carb differences between tomatoes. It's really very tiny, and since every single tomato is different than its brothers, it's best just to use the standard count as an average across all tomatoes you eat.
Vegetable Carb Chart
Lisa Shea's Library of Low Carb Books