There's nothing quite as satisfying as rebuking someone making fun of you in a language they don't think you can understand. Being multi-lingual is an immensely beneficial skill (at least until we're all implanted with Babel fish).
Looking to attach another language to my resume, I found the language teaching service Duolingo. The website takes English-language-users through the basics of other tongues (currently limited to German, Spanish, French, Portuguese and Italian). This is done through a series of tests including both written and oral components, which become more difficult as you progress along a skills tree.
New and more difficult language modules can only be unlocked by gaining experience in previous levels. Upskilling earns badges and other digital knickknacks which you can compete for against your friends. This gamification is oddly satisfying and fools you into forgetting you're learning a new and useful skill.
Duolingo is free. Completely and utterly free. No ads, no freemium model, no sales person hiding in the corner waiting to sign away the soul of your firstborn child. Free. The service is built by the same folk behind reCaptcha, which protects websites from spam while providing text translation services to businesses. Duolingo works in a similar way. People needing translation services upload documents to the site and then Duolingo users translate portions of it while doing lessons. If enough people work on the document it effectively brute forces an accurate translation, which Duolingo is then paid for by the uploader.
The best part of the Duolingo experience is its seamless shift to mobile. The iOS and Android apps (for tablet and smartphones) are excellent clones of the web version and once again completely free.
I don't see Duolingo as a complete replacement for language tuition, but it is a great first step towards some competency in a new language. This is the kind of app you can give a child or teenager and they'll be able to really excel in their language classes at school. Or something a grown up can tackle in short periods of downtime to pick up a few things about different languages.