Wikipedia told me: The tomatillo (Physalis philadelphica) is a plant of the tomato family, related to the cape gooseberry, bearing small, spherical and green or green-purple fruit of the same name. Tomatillos, referred to as green tomato (Spanish: tomate verde) in Mexico, are a staple in Mexican cuisine. Tomatillos are grown throughout the Western Hemisphere."
Sounded like I should put it in some kind of salsa or guacamole, and I figured my usual mix of veggie, spices, herbs and oil should do the trick. I served it with blue corn tortilla chips, and the resulting recipe was tart, tangy, and had a delightful mix of textures. A must-try if you have tomatillos on hand for some reason.