Fruit is a food group that can be confusing to low-carb eaters and diabetics. It is also an area where some of the popular low-carb diet plans differ, as some depend more upon glycemic index or glycemic load (South Beach, Zone), while others just look at the amount of carbohydrate (Atkins, Protein Power). Also, some diets (Atkins, South Beach) don't allow fruit at all in the first phase. In general, your best bet when you eat fruits are to try the ones near the top of this list, but do check carb counts by clicking on the fruit you are interested in. This list is roughly arranged by sugar content according to the USDA database, based on a half-cup serving of fruit.
You may hear that you don't have to worry about sugar in fruit, because it is natural to the fruit. The truth is that "it depends". Certainly, fruits contain many nutrients, and if you're going to be eating sugar it's much better to have some great nutrients to go with it! On the other hand, some bodies handle sugars better than others, and if you are someone who responds well to a low-carb diet, it pays to be careful.
If possible, check your blood glucose to see how the fruit (or any food) affects it.
Good news: the fruits lowest in sugar are some of the highest in nutritional value, including antioxidants and other phytonutrients.
If you are considering buying organic fruits, check out this list of which fruits and vegetables have the most and least pesticides to help you guide your choices.
Fruits Lowest in Sugar
Small Amounts of Lemon or Lime
Raspberries (more about berries on low carb diets)
Fruits Low to Medium in Sugar
Guavas -- Pineapple Guavas (Feijoa) and Strawberry Guavas are probably similar, but information that directly compares them is not available
Fruits Fairly High in Sugar
Fruits Very High in Sugar