Grammar
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EspressoEnglish has lots of free grammar exercises and explanations.
The British Council offer a variety of grammar exercises for different levels.
Reading
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Lingua.com has reading comprehension exercises for a variety of levels.
Breaking News English is great if you are interested in current affairs.
The British Council has a great selection of fiction stories for all levels.
Listening
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Ted.com talks and presentations on a large variety of topics.
TedEd the same as Ted, but with comprehension questions to check understanding.
BBC Sounds Listen to BBC radio and podcasts.
NPR US news and podcasts, on a large variety of subjects.
British Council Podcasts - great for low levels.
News In Levels - great for low levels, choose “Level 1”.
Speaking
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The Tandem app is a really nice way to connect with others, and improve your language skills.
Conversation Exchange also puts you in touch with other learners.