Because English is not my native tongue, my grammar is as good as I would like it to be. While nothing is going to replace (at least for now) the human brain when checking grammar, some tools may help.
I've learned that TextEdit (the default text editor in Mac OS X) comes with grammar checker. While it is not good, it may help.
To enable it go to:
Edit -> Spelling and Grammar -> Check grammar with spelling
Once done, you can write your text, and when you are done, select:
Edit -> Spelling and Grammar -> Check document now