Duolingo is a language-learning website which offers courses for speakers of many different languages, including English courses for speakers of Arabic, Thai, Hindi, and many others.
Lang-8 is an interactive language exchange website which allows language learners to post text in the language they're learning, and receive feedback from native speakers of that language.
English My Way is a BBC website which has dozens of free language modules specifically for English learners. Topics covered in the first module include going to the dentist and taking the bus.
This website has lots of different childrens books, translated into multiple languages. It is a great resource for parents, and for adult language learners who want to start with the basics!
Simple English Wikipedia contains articles written in basic English, with shorter sentences and easier words. It is specifically designed to be used by people who speak English as a second language as an educational tool.
This free series produced by the British Broadcasting Corporation will take you through sentence structure, vocabulary, and phrases used in modern English. An intermediate course is also available for those who want an added challenge.
A slightly older, but highly rated series produced by the Center for Educational Development in Los Angeles, California. The podcast includes many words commonly used in North American English as opposed to United Kingdom English.
A culture and idiom-heavy podcast for advanced English learners produced by the British Broadcasting Corporation. Recommended for learners who are approaching fluency and want to sound more natural when they speak English.
A podcast containing unscripted conversations in English, featuring a wide variety of people. Useful for intermediate learners who want to improve their listening comprehension.