For the past four weeks I’ve been obsessed with the new language learning tool, Duolingo. Developed by Luis von Ahn, a professor at Carnegie Mellon University (the guy that brought us reCAPTCHA), Duolingo was named by Apple as the best new iPhone app for 2013. (There’s a web version as well.) Slate writer Seth Stevenson calls it “the most productive means of procrastination I’ve ever discovered.”

Oh he is so right.

Duolingo is a great example of game-driven learning. A little green owl guides you through short lessons that are less about conjugating verbs and more about watching those verbs perform in easy-to-understand sentences. As you learn, you’re incented to win the game. With each achievement, a trumpet blares and the green owl shows you a graph of your daily progress. You earn “lingot” bonuses when you make it through a checkpoint or finish a lesson without making an error. You can also opt to play with friends from the Duolingo community or just eavesdrop on their discussions when they pick apart a sentence. Duolingo has over 5 million daily users with an estimated 100,000 joining daily.

It’s all rather silly, but crazy effective. And here’s the best part: it doesn’t cost a cent, it’s ad-free, and, von Ahn promises, it will remain that way. As he explained in a TED talk a few weeks before launching Duolingo, his audience really isn’t me. His real passion is helping people who can’t afford the $500 license fee for a program like Rosetta Stone learn new language skills that will give them a leg up in life. What’s astonishing, heartening, oh my god almost makes you believe in entrepreneurialism again, is that he’s found a revenue model to make this possible.

Because von Ahn’s other passion is to translate the Web into every major human language. Not by using machine translation (which he estimates won’t be up to the task for another ten to fifteen years), but by crowdsourcing that effort with Duolingo addicts like me.

This morning, to try out my new Spanish skills, I clicked on the “Immersion” tab in Duolingo and chose a half-translated web document about Arthur Conan Doyle’s “The Hound of the Baskervilles.” I took a look at a short sentence that had been translated by my Duolingo peers and changed the descriptor of a secondary character (“corpulenta” in Spanish) from “bulky” to “portly.” A good edit? Who knows, but because it’s crowdsourced, the change will be voted up or down and eventually a “final” version will be posted.

These translation services, which people like me (happily) perform for free, are sold to organizations like Buzzfeed and CNN. Biting a chunk out of the $30 billion market in translation services could potentially make Duolingo a sustainable–and free–learning tool for years to come.

Hats off, Professor von Ahn! Duolingo is a finalist for best education startup in the 2014 Crunchies Awards being held on February 10. Ed-tech vendors take note: you really can generate the revenue you need to run your businesses without giving up on trying to change the world.