According to its Wikipedia entry, Esperanto is a constructed international auxiliary language that up to 2 million people worldwide speak, but that has as
few as 200 native speakers. Esperanto was conceived by Dr. Ludovic Lazarus Zamenhof, a Russian Jewish opthamologist, who published the first book on the language in 1887. Zamenhof sought to develop an easy-to-learn language that would serve as a universal second language to foster peace and international understanding. It’s estimated that Esperanto can be learned in anywhere from one quarter to one twentieth the amount of time required for other languages.
To date, no country has adopted the language officially, but Esperanto can be found worldwide in correspondence, cultural exchanges, literature, television, and radio broadcasting. And Esperanto has an online presence: Google lets you search Esperanto-related websites via an Esperanto portal, and the Esperanto Wikipedia contains over 118,000 articles, as of September 2009.
If you’d like to teach yourself Esperanto, check out the website lernu!