For practice with grammar and punctuation, try a site like Every-Day Edits from Education World:
www.learningreviews.com/jreviews/Languag...Every-Day-Edits.html
If you need someone to read your writing to help you improve it, connect with other language learners on a site like English, baby! (in their Find Friends section):
www.learningreviews.com/jreviews/Languag...LL/English-baby.html
Another helpful site is LiveMocha. Some of their lessons are free. In their "Share" section you can make friends and exchange writing and speaking exercises - you write or say something and they correct it, then they write or speak something in your native language and you correct it:
www.learningreviews.com/jreviews/Languag...uages/LiveMocha.html
For more help, look at the entire collection of English Language Learner (ELL) sites:
www.learningreviews.com/jreviews/English...age-Learner-ELL.html 
