I use Open Office (well Libre Office) and actually the grammar checker isn't the best or even nearly as intuitive as the one in MSWord. There are several others like Grammarly, which is probably the best (which isn't free even though it's listed as free on Google).
I use Pro Spell Checker plus, but unfortunately it makes a lot of its own errors and asks a lot of questions that could make checking even more confusing and difficult for a non-native English writer. Questions such as "Did you mean your or you're?" and errors such as "It's inappropriate to start a sentence with 'because' ?" (a quite antiquated rule and not a great one for the web.)
I also use Paper Rater, which is pretty good, but again, I have never found an automated grammar checker that didn't make errors of its own.
The best thing you can do is use a look up reference like Owl of Purdue to help you know what's the right thing to do in the first place.
We also have an entire thread already devoted to [grammar resources plus another that is full of [URL="http://www.sitepoint.com/forums/showthread.php?529530-Online-Web-Content-Writer-Resources"]good resources for writers](http://www.sitepoint.com/forums/showthread.php?523640-Any-online-guides-on-improving-your-Grammar). Please take the time to read through them. Then, if you have a specific question that hasn't been answered, come back and post it. However, for now, this thread is closed.