Writing Center

provided by "Accredited Colleges"

This is a very useful resource on:

grammar
punctuation
other common (writing) issues
different writing styles
citation guide.

While the target audience is college, the text is easy to read and understand. It is definitely helpful for grades 7-12, but even upper elementary students will find it useful.

The direct URL is http://www.accreditedcolleges.com/writing-center/