Post in the language you're learning, and get advice from native speakers.
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Pro Gain experience writing paragraphs
Where many programs will have you writing single sentences at most, Lang-8 encourages you to write short stories or diary entries.
Pro Get feedback from native speakers
Native speakers of the language you are learning will give you pointers on your writing, such as grammar corrections and advice on word choice/sentence structure.
Con Not a good starting place for beginners
The site doesn't teach you your language, instead it allows you to get advice on your writing in your target language. It's better if you have a basic understanding of the language at first.