When I am writing, proofreading or copy-editing these are the places I turn to, as well as my own library of ‘real life’ books, to make sure I’m on the mark.
If you want to improve your writing skills, aside from using English Pro’s forum, these are excellent places to start:
Common mistakes explained: http://www.dailywritingtips.com/
Writing guidance for copy-writers http://www.copyblogger.com/
Masses of information about writing http://owl.english.purdue.edu/owl/resource/681/01/
Easy guide to punctuation: http://www.informatics.sussex.ac.uk/department/docs/punctuation/node00.html
And if you’re still learning English, these guys have some fabulous, interactive games: http://www.englishgrammarsecrets.com/index.html