An evolving resource for web tools and other resources for language learning and teaching that includes a wide range of free and freemium tools that can be easily searched and browsed by skill, type of collaboration possible, and cognitive level (based on Bloom's taxonomy, terms from www.usi.edu/distance/bdt.htm). Limitations are listed in the comments.
A free online course with featuring six units with full audio dialogues and accompanying activities. Users gain exposure to reading and speaking Arabic. This resources also introduces users to Arab culture. This is good practice for anyone who needs some extra help speaking the language.
Use books and articles in English or import your own in any language. You can add annotations and questions to the articles, plus create quizzes. Students can also comment on the articles.
Limitations: Free accounts can only import 3 articles per month. Presumably, you can use as many of the existing articles and books as you want, but they are only in English.
List of free online courses, including a few that may be useful to learners of English, e.g., "Study Skills for International Students" and "Exploring English: Language and Culture."
Gamified Japanese language learning. Includes hiragana, katakana, kanji, vocabulary, and news articles. You earn experience points by correctly answering questions. Also includes social features and a dictionary.
Online Speech Recognition in many languages. Text can be exported.
I tried this with English and Japanese, and it quite accurately transcribed my speech, except proper names.
Teachers could use this to help students improve pronunciation.
Dictionary that offers definitions, synonyms, antonyms, example sentences, pronunciations, and translations between English and several languages (German, Dutch, French, Italian, Portuguese, and Spanish). Many other options.
Portuguese language lessons that highlight aspects of Brazilian culture. These lessons are designed for intermediate to advanced students, but are accessible to everyone. Each includes videos of Brazilians from all walks of life speaking naturally about their lives and their country, and numerous activities and exercises available in PDF files.