Lang-8 is a website where you can write a journal post in English, and have your entry corrected by native speakers of English.
In return, you're encouraged to correct journal posts written in the language that you're a native speaker of.
(Declaration of interest: I'm a user of the site) Update: It is shutting down
Italki is another great language exchange website, and there you will find many native speakers who will gladly and eagerly correct sentences or even short essays. On the how it works page, it states:
CORRECT YOUR WRITING
Write short essays using a foreign language and get corrections from the community. Share your ideas and opinions while improving your writing skills.
I [@GoDucks] use that site from time to time.