If you measure stock exchanges by how many stocks they have listed, the smallest stock in the world is the Douala Stock Exchange also known as the DSX, based in Cameroon. This exchange has had only only one listing since its founding in 2001, Société des Eaux Minérales du Cameroun (SEMC). However, in May of this year, they just added their second issue, Société Africaine Forestière et Agricole du Cameroun (SAFACAM). The exchange also trades government bonds, however those are considered unlisted securities.
There are a lot of interesting exchanges around the world, including exchanges that trade carbon emissions. One is the Climate Exchange.