The Blue Book of Grammar and Punctuation
"Use this site to find the answers to your questions concerning proper English grammar and punctuation."

Guide to Grammar and Writing (via Capital Community College, Hartford, CT)
This online guide answers many questions students have about writing, from fine points of grammar and paragraph structure to essay organization and research paper citation formats.

The International Writing Centers Association (IWCA)
This site contains information for students, teachers, and tutors.  The "Resources for Writers" section includes many  links to sites that can help students with their writing projects.