Created by Lazar Zamenhof, a Polish oculist (!), in the early part of the 20th century, Esperanto is certainly the world's most well-known 'auxiliary language'. Use of the language has spread world-wide in the century since its creation. In spite of its strong Indo-European base it is very popular in China and some other countries in the Far East.
Opinions vary as to how many Esperanto speakers there are. Esperantists quote numbers in the millions. Non-Esperantists are inclined to quote numbers in the lower hundred-thousands. Much probably depends on how you define 'speaker' -- the number of truly fluent Esperantists is going to be much lower than the number of those who just know the basics of the language.