Of all the free language learning websites I've looked at, Duolingo is probably the easiest one to use.
The website is friendly and interactive to make it both simple and fun to learn a new language. In addition to text, Duolingo utilizes your microphone and speakers to teach you how to speak and understand other languages.
Languages You Can Learn at Duolingo
Visit the Courses page at Duolingo to see which languages you can learn
For example, German speakers can only learn English, French, and Spanish, whereas English speakers can learn all the languages listed below (excluding English).
Danish, Dutch, English, Esperanto, French, German, Hungarian, Irish, Italian, Klingon, Norwegian, Polish, Portuguese, Romanian, Russian, Spanish, Swedish, Turkish, Ukrainian, Vietnamese
How Duolingo Works
There are dozens of sets of lessons at Duolingo called Skills. Some of them include the following: Basics, Phrases, Food, Present, Adjectives, Plurals, Family, Questions, Numbers, Household, Colors, Comparison, Qualifiers, Prepositions, Dates & Time, Nature, and Medical.
The lessons include images, text, and audio, and sometimes has you speak into a microphone (if you have one) to test your speaking and pronunciation skills.
You can take each lesson one after the other to gradually move into more difficult concepts, or you can choose Test out to take one quiz that combines a little of each lesson into one big test.
A couple test out options are available for a handful of skills. If you pass a test out, you can move over all of those lessons and start somewhere a bit more advanced.
Because Duolingo works this way, it can be beneficial for both someone who needs to brush up on their language skills as well as someone who's brand new to the language.
An Immersion section of Duolingo's website has you translate real articles on the Internet. You can also correct the translations done by other users. As you learn new words with Duolingo, they will be stored in the Words section of the site for you to access and review any time you need to.
You get points as you complete tasks, so you can setup a daily goal to require that you can reach a certain number of points. This helps you stay focused and determined to reach your daily goal.
A user account isn't required to use Duolingo, but it's recommended if you want to keep track of your progress.
Duolingo also has free mobile apps for learning a new language so you can take the lessons with you wherever you go.