I saw the basic recipe for this in Bon Appetit magazine, and was intrigued by it. So I broke out the blender and gave it a shot. Pay attention to the notes at the bottom, please.

Basic Recipe

4 cups of fruit, cut into 1" pieces, frozen (I used pineapple; other suggestions include mango, watermelon, or honeydew)
3 tbsp fresh lime juice
1/2 tsp kosher salt
2 tbsp agave nectar, or 1 tbsp honey or maple syrup
2-3 cups of water (or coconut water)

Blend all ingredients together with 2 cups of ice until smooth, adding more water or coconut water as necessary, until it's smooth. Taste it and add more agave if desired. Divide among glasses (it serves 4).

Now, this is how I jiggered it for just myself:
1 cup pineapple chunks, frozen
2 tsps lime juice
2 tsps sugar
pinch kosher salt
1/4 cup water
1/4 cup crushed ice

Note 1: To spice it up, the magazine recipe suggested adding 1 tbsp of finely grated ginger to the pineapple. I added 1 tsp of minced ginger. Spicy! Next time, I'll cut it back a tad.
Note 2: Overall, it wasn't bad, and a great way to get some fresh fruit.
Note 3: I figured that this amount was about 92 calories (just to be sure, I rounded up to 100), with no fat. And nice and chill for a summer afternoon.