Improving your accent is a different ball game alltogether. As far as writing skills are concerned, just try to read articles on reputed sites. There is a vast difference between ordinary writing and content writing and you hone your skills as you keep on writing and learning from your past mistakes. Just write one article and scan it later on, you might come up with an idea to improve it by juggling with some words or phrases.
As far as software is concerned, I have tried `whitesmoke' and was not satisfied. It could not catch simple errors that `MS Word' has no problems trapping. There is one site on the net where you can take different English tests, I do not recall the same offhand though.
