One of the most underknown vegetables (maybe along with daikon radish and kohlrabi). You've probably all eaten it without recognizing it, because it's a stable ingredient in Mexican salsas.
But, these tomato relatives are fabulous in a wide variety of cooking. When I buy them at my normal grocery store, most clerks (except for Irma, who is from Mexico) are mystified by them. They are both inexpensive and easy to use, so I hereby recommend my personal usages:
1. In any form of oriental stir-fry dish.
2. In pasta primavera dishes.
3. As a garnish on baked fish.'
4. On pizza. I frequently get basic uncooked pizzas from the Papa Murphy's chain, and add stuff to them, including tomatillos.
5. Chopped very fine, and added to rice pilafs or similar dishes.
6. On grilled hamburgers. One of my favorites is to slide the tomatillo as you would a tomato, nestle it in a ring of bell pepper, put that on the burger as you grill, lay a slice of your favorite cheese over it, and place on that some finely diced jalapeño pepper.
They are like slightly tangy, slightly salty and less sweet tomatoes. Try them.