The company behind WordPress has written a pretty amazing online grammar and style checker.

It integrates with WordPress and other platforms, providing really good suggestions.  Each mark also comes with an “Explain” option.  That’s nice because it gives you the reasoning for the suggestion.  I’ve installed the WordPress plugin – expect less passive voice from me.

If you’ve got a blog where you’ve got any control over it at all, I’d suggest you try the demos, then integrate it into your site.