Salmiakki (Salmiac), NH4Cl, also known as ammonium chloride and sal ammoniac.

As told above, it's used in candies in Finland; My personal favorite is Super Salmiakki from Fazer.

Fazer's Salmiakki was first marketed by them in 1939. The Salmiakki candies are traditionally rhomb-shaped (to the extent that "salmiakki" also means a rhomb shape in Finnish!). Super Salmiakki candies are round and flat, but have rhomb pattern imprinted on them.

Finland even has a society of Salmiakki lovers (Suomen Salmiakkiyhdistys Ry).

Salmiac is produced by chemical reaction between ammonium and hydrochloric acid; we actually made bunch of this stuff in chemistry class in high school, and the teacher let us taste it. (The paper on which the salmiac powder was on was completely spotless when people came back from break... =)

This stuff also has some other uses as a chemical. It can be found in batteries, for example. (Some brave souls apparently ate salmiac from there. Brave. Really brave. Kids, don't try that at home unless you know what you're doing. These days, batteries also probably have some not-that-harmless stuff. =)