Recs.
Updated
Duolingo is a website where you can take courses in several languages. Each lesson uses a variety of different learning methods to help make sure the lessons stick.
SpecsUpdate
Pros
Pro Motivates learners through gamification
Progress is measured gaming-like by gaining XP, leveling up and knowing how many words are learnt. Also, lessons have a limited amount of lives, which you must preserve to pass the lesson. They use other creative gamification techniques to keep you motivated, such as making wagers and improving your position on the leaderboard.
Pro Frequently adding new languages
You can check out the courses page to see what languages are "hatching" (being developed) and what languages are in beta.
Cons
Con The hype in the community creates false ideas about what level Duolingo gets you to
Duolingo is a good tool for a beginner, and a good supplement to other resources. But it cannot get you from zero to understanding natives, tv, and books; and their "do the reverse tree and just speak" is usually not the correct answer to "what should I do after finishing the tree".