Esperanto

Esperanto is essentially English, French, German, Portuguese, Swedish, Danish, Norwegian, Italian, Spanish, Dutch, Polish, Czech, Finnish, Japanese, Mandarin, Thai, and Russian all divided by 18. Will McGree (in John Cowan's Essentialist Explanations)

Esperanto is a constructed “international auxiliary language” that was created by the Polish ophthalmologist Ludwik Lejzer Zamenhof in the late 19th century. Esperanto was not the first constructed language; for example, Solresol and Volapük were also created in the nineteenth century. However, Esperanto is the most successful constructed language. According to the most optimistic estimates, there may be as many as two million users of Esperanto worldwide.

Esperanto was designed to be easy to learn. It has a regular (and easy) grammar, and its vocabulary is easy to learn for speakers of European languages. It is also an easy stepping stone towards other foreign languages (see The Propaedeutic Value of Esperanto below).

The creation of auxiliary languages still continues. More recent examples include Ido (considered the most successful offspring of Esperanto), Afrihili (which is meant to be easy to learn for Africans) and Lingwa de planeta or Lidepla (based on the most widely spoken languages of the world). See also the YouTube video Why People Make Their Own Languages by Xidnaf.

Not all constructed languages are auxiliary languages. For example, the languages of Middle Earth created by Tolkien and the languages used in George R. R. Martin's A Song of Ice and Fire (Dothraki and Valyrian) were created as parts of the fictional words invented by these authors and are not used in the real world (except perhaps in fan communities).
Another well-known example is Klingon. (Someone even tried to raise his son as a native speaker of Klingon.)

Don't learn Esperanto!”: a humorous video on reasons for (not?) learning Esperanto.

Resources for Learning Esperanto

Before the arrival of the Internet, most people learnt Esperanto from books and later from self-study courses accompanied by audio materials. The World Wide Web has given the Esperanto community more convenient ways to learn the languages and to communicate with other learners and Esperantists. Many learning materials are now available on the Web.

Dictionaries

Materials in Esperanto

Esperanto: What & Why

The Propaedeutic Value of Esperanto

Other Reasons for Learning Esperanto

Other Information about Esperanto

Esperanto Assocations