Esperanto is the most widely-spoken constructed language in the world. It was created by a Polish-Jewish ophthalmologist, Dr. Ludwig Lazarus Zamenhof (1859-1917). He published a book about the language in 1887. It was designed to be very logical and easy to learn, so people who speak different languages can communicate with each other more easily.
There are as many as 2 million Esperanto speakers worldwide. 1,000 - 2,000 are native speakers.
The Esperanto Map
The map of the world above is coded by color. The colors correspond to countries by number of members of the UEA (Universala Esperanto-Asocio) or Universal Esperanto Association. The amount corresponds to the chart and color key below. Basically the countries that are darker green have more Universal Esperanto Association members.
1. 1-9 members
2. 10-49 members
3. 50-99 members
4. 100-199 members
5. 200-499 members
6. 500-1600 members
Most countries also have their own Esperanto organizations which have many more members.
Source for map and key.
Larger version of the Esperanto Map.