A spicy Vietnamese sauce. As I make it, it includes:
1/3 cup Fish Sauce (nuoc mam)
1/4 cup rice wine vinegar
the juice of 2 limes
5 teaspoons sugar
1 minced scallion
1 pressed clove garlic
1/8 cup crushed roasted peanuts
a generous squirt of chili sauce (I use Sriracha brand, with the rooster on the bottle)
It's a good idea to let the sauce sit in the fridge for several hours before serving. It blends the flavors much better.

Note that there are many different variations of this sauce. Put in it what you like!