lang-8.com is well known amongst intermediate/advanced language learners who are self-taught, but I thought I’d explain it for beginners. It is an amazingly useful site where you can blog or upload any text you’ve written in the language you’re learning. Many people use it as a diary, though I’ve seen people check essays and business emails there too.
Once the text is uploaded native speakers of the language will check it for you. You return the favour by checking pieces of work written in your native language and correcting them. In this way everyone gets their work marked by a native speaker (often more than one person contributes so you can get several opinions too).
It is massively helpful in improving all aspects of writing, but natural usage in particular (notoriously difficult for self learners to pick up when they are not in a native environment).
Wow this page is AWESOME