C: comunication

C: cognition
C: culture 

Who would love it: First-time learners as well as veterans looking to brush up on their skills before a trip; especially friendly for younger audiences.

Why it's good: Most free language-learning apps can feel like a random stack of unrelated flashcards. Duolingo actually features an intuitive, step-by-step learning progression that leads speakers through the basics of the language while gently challenging them and explaining key grammatical concepts. For that, and its excellent design, it’s also been named one of the best apps by Apple, Android, TechCrunch and prof. Di Lorenzo.

Why it's relevant: Duolingo has a track record of excellence in design and education. It is also expanding, adding new languages and greatly improving its Web experience. It may not be as robust as a Rosetta Stone, but I’ve found it to be more engaging and a joy to use (not to mention: free).