Esperanto
 

Esperanto is an international auxiliary language (an IAL) created by Ludovic Lazarus Zamenhof and first published in 1887 in the Unua Libro (First Book). He first called the language simply La Internacia Lingvo and envisioned it as the one international language that would solve many of the world's problems. He signed the book as Dr. Esperanto, using as a name a word meaning "one who hopes". The name Esperanto was then applied to the language and has been used ever since.

And while it may not solve all of the world's problems, Esperantists do seem to be a pretty hopeful bunch. The language, for me, is mostly a matter of academic and recreational interest. I can communicate now with an estimated 100,000 to 2 million Esperanto speakers, many of whom speak no English. This isn't a huge number for a language, but they're spread all over the world in a huge number of countries.

Of course it's useful, but Esperanto can also be fun, as any language should. There's a wealth of literature (albeit not always very handy), and a few online communities. And as a conlanger myself, I have fun with it just in itself. For example see my page on Writing Esperanto in Greek.

I will probably put more information here eventually, but check out Wikipedia's Esperanto Portal in the mean time. Interested in learning Esperanto? Take a look at lernu.net.