I saw them at an outdoor display beside bins of dried squid, mussels, shrimp, and fish, in various sizes and opacities; they had the size and shape of kiwis but little more heft than ping-pong balls. Their sign, you'll note, bore only Chinese characters; I could make out "three for a dollar," but no one was available to translate the rest. A query on Chowhound generated a flurry of replies and identified them as Siraitia grosvenorii, more commonly called luo han guo, sometimes monk's fruit or arhat fruit.
Extracts from luo han guo can be used as a concentrated, low-calorie sweetener, but the dried fruit is more typically used to prepare an infusion that soothes sore throats and, traditionally, any number of other maladies. The counterwoman and a customer recommended that one-quarter of the fruit be used for each cup, though as you can see from the photos below, cutting open the luo han guo's brittle shell shatters it rather than quarters it. Steeping the dried fruit, then straining out the fragments, is one possibility, but it's a much more elegant solution to make your own "tea ball" and prepare a potful.
158 Hester St. (at Mott St.)