Duolingo is one of the most popular free language services. It's currently available in only about 20 mostly European languages and is not available in Arabic or Chinese. You must create a free account to begin. Duolingo is popular because of its easy-to-use-layout and short lessons. Users can connect with other language learners and compete against friends, and the game-like atmosphere of Duolingo makes it fun to use. Duolingo is also available as an app for iOS, Android, and Windows.
Italki is an online community and marketplace for language learners. It makes it easy to find language partners for a free exchange, in which you speak in your native language to help your partner improve for half the discussion and then speak in your target language, your partner's native language, for the other half. Though joining the community is free, many users also offer their services as language teachers for a fee. Italki provides scheduling tools to make language exchanges easier.
Quizlet is a popular online flashcard tool, which requires a free account to use. Once you've created an account, you can either find "study sets" (sets of flashcards) created by other users, or you can create your own. Flashcards can be created in any web-compatible font, and it's also possible to add audio to the cards. Students can share cards with classmates, and many study sets exist for common foreign language textbooks. Quizlet offers a variety of ways to study: a traditional flashcard system; learn mode, which requires you to type the response, thus making you more accountable; spell mode (for cards with audio), in which you hear the word and then type it; test mode, which allow you to create custom tests out of flashcards; scatter, a matching game; and gravity, an arcade-style game. Quizlet is also available as an iOS and Android app.
Forvo is an online pronunciation dictionary where users record words or phrases in their native language to share. Native speakers provide information about their language background. With a free account, you can download recordings to add to your online flashcards (lots of study sets on Quizlet use Forvo recordings, for instance) or request pronunciations of certain words.
Transparent Language Online helps you learn over 110 different languages. You can learn English from 30 different languages. Lessons for kids too!