I think you're looking for a website with a lot of writing work which will improve the grammar right?
Well, if yes, then I generally use http://www.studymode.com/
to help my students with writing ideas and topics. My students find it very helpful and they read up on the essays, research papers, and other UGC. It has improved their grammar skills and thinking capability.
If not, you can also visit some other sites like Khan Academy etc.