Duolingo is completely free with no ads or hidden fees. The reason its free is that the users create value by translating real-world documents while learning. Essentially, somebody who needs a webpage translated uploads it to Duolingo. That document then gets presented to Duolingo students who can translate it in order to practice the language they are learning. When the document is fully translated, Duolingo returns it to the original content owner who pays for the translation. While this might sound dry and boring, Duolingo is actually designed as a game (and thus is very popular as a smartphone app), which at least one reviewer seems to find quite effective.