With such easy access to technology there are so many ways that we can make our (and our students') writing better with minimal effort. Here is a list of a few resources that are my "go to" sites when working on writing something.
1) http://www.visualthesaurus.com/
I absolutely LOVE this website. It is, as the name suggests a digital thesaurus that allows you to visualize the connections between words. After having stumbled upon this site I don't think I can ever go back to a "traditional" thesaurus again.
Besides using it as a means for the students simply to look up synonyms to words they use in their writings, this site can be used as a tool, perhaps projected overhead, as a means to talk about the relationship that words have with one another and maybe explain why in some instances 1 word would make a good replacement and in another instance that same word might not work.
2) http://www.dailywritingtips.com/
This is another "go to" site that I use for general questions regarding correct word usage and grammar. I have also been there to get answers to things that are hard to explain, such as the difference between effect and affect, or toward vs towards.
3) http://www.duplichecker.com/
This is website is meant to check for plagiarism. You enter the excerpt that you are curious about and confirm that you have not plagiarized
This is a great tool to use for student's to check their own work if they weren't sure if the excerpt was changed enough so that it wouldn't be plagarim
4) http://www.toolsforeducators.com/writing/
This site provides different worksheets and flashcards that can be used to help students learn and understand. I think the students could use it as a leaning tool and work on vocabulary that can help their writing.