Inspired by 
In the end of the 19th century it became increasingly clear that the world needs to be considered as one unite and only joint efforts could solve the world's problems. International institutions popped up like The International Telecommunication Union (1865), The Universal Postal Union (1874) or The Permanent Court of Arbitration (1899). Different attempts were made to get an constructed international language working. Esperanto being the most successful of them over the years developed to an natural language spoken by some millions around the globe.
Two years after the first World Esperanto Congress in France in 1905 the mathematician Dr René de Saussure proposed a new currency called Spesemilo. It was made for international money transfer for Esperantists and issued as vouchers by few British and Swiss banks, mainly by the Ĉekobanko esperantista. The Ĉekobanko was founded in 1907 by H.F. Höveler in London with branches in Dresden and Moscow. On their checks was to read the slogan "Unu mondo, unu lingvo, unu mono" (One world, one language, one currency).
In 1914 existed 730 accounts in 320 cities in 43 countries. This currency was used until the outbreak of the First World War. One Spesmilo (from German word Spesen/expenses and milo/Esperanto thousand) was equivalent to one thousand spesoj (penny), and worth 0.733 grams of pure gold which at the time was about one Russian ruble or one-half US-dollar. On 19 January 2014, that quantity of gold would be worth about $33 U.S. dollars. The basic unit is the speso.
Today you can find Spesemilo as Unicode character (spesemilo sign U+20B7).
Silver coin issued in 1912 for the 25th anniversary of the first publication of Dr. Esperanto's International Language in 1887.
In 2011 one of the still existing four coins was auctioned for $261.
With the beginning of the First World War interest in an international currency rapidly disappeared, however after World War II with the foundation of The Universal League in the Netherlands (arm of the Esperanto movement) a new attempt was made to revive the idea of a common currency.
The new token was called Stelo (star), and used from 1945 to 1993. The Universal League issued coupons denominated in Steloj. The idea was to link the Stelo to existing currencies on the basis of relative purchasing power in different countries. The value of a Stelo would equal the price of a standard loaf of bread in the Netherlands, which at that time cost 0.25 Dutch guilders.
The League had commercial relations 3 banks in the Netherland as well as with banks in six European countries.
In 1959 real coins were minted in bronze, copper and nicle by the Royal Dutch Mint in Utrecht. They sold well so that in 1965 the League was able to issue a fourth denomination, a silver 25-Steloj coin, which also sold very well.
An interesting point about this currency is, that out of its use came an other idea. Some members of the League like Dr Roelofs believed that the Stelo should serve as the basis for international financial transactions, but only between countries that refused to deal with commercial banks. That sound quite familiar today.
However his ideas never really became reality, and with the ending of The Universal League the token disappeared too.
On a much smaller level the Stelo in a new version survived in the Esperanto movement. Nowadays exist three plastic coins 1★, 3★ and 10★. They look a bit like game money and are mostly used in meetings and congresses by younger esperantists. The value of 1 Stelo is the equvalent of one cup of tea.
In 2012 French-Danish artist Daniel Salomon issued an new currency called Mono (money) which he claims will be one day the only currency on earth. He is the director of the Sennacia Banko (Bank without nations). In a kind of art performence he not only spread his idea of a common world currency, he also gave away 100 mono paper notes, which show the picture of L.L.Zamenhof, initiator of Esperanto, on one side and a snail (a reference to the time this project needs to develope ?) on the other. The notes are beautifuly produced by a commercial currency printer in Asia.
With the appearance of cryptocurrencies, specially Bitcoin, the dream of a worldwide usable autonomous and decentralized currency could finally become true. On the Internet are a lot of discussions about this, and one enthusiast already created on the Counterparty's platform a new coin EsperantoXCP to commemorate the idea of a World´s Unique Currency. He also revived the Stelo and hopes that both will win great support.
So I am very curious to see what comes out of such initiatives. What do you think about a special token for Esperanto speakers? Would a own currency really help promoting a global language?