Here's an interesting and ambitious crowd-sourcing venture. It's called Duolingo and it's from a team of computer scientists, designers and linguists based at Carnegie Mellon University, Pittsburgh, USA . You sign up for free - no ads, no later subscriptions, and you learn a language by translating it into your own at a level that suits you. Languages available include French, Spanish, Portuguese and German. In so doing you also help translate the web for the good of everyone in the world.
"The Service allows users to learn or practise a language while they translate content from the Web. Users are presented with different types of educational activities; while they perform these activities, they also generate valuable data such as translations of Web content."
Here is their short video which explains how it works.
There is an iPhone app so you can use the method on the go. They have had an independent eight week study done by two academics in December 2012 which shows how effective it can be.
How is it free? They say:
"Some language learning websites claim to be free, when in fact they are only free to use for a limited time. Duolingo has a different philosophy. Our language learning tools are completely free, because while you are learning you are also creating valuable translations for the world. This unique setup means that Duolingo is offered without ads, hidden fees, or subscriptions—Duolingo is 100% free, today, tomorrow and forever."
(Update: in a Guardian article the makers explain how they are motivated by providing free access quality education to all, irrespective of income).
They have benefited from some external investment and I suppose they will be able to sell the fruits of learners' labours. I do note that they will have information about you when you register. Am I just being too suspicious?
Anyway, this looks very good for adults, and maybe it could be recommended to students. They only have to sign up. Here is more information if you are interested. Here is a revie from PC Mag. And here is one person's experience.