Capital Community College in Hartford, Connecticut, has made an excellent Guide to Grammar and Writing available over the Internet: http://grammar.ccc.commnet.edu/grammar/index.htm.
When I had to train copyeditors and proofreaders, I would have them spend their first few days on the job reading the material on this site and taking all the quizzes. If you were never taught grammar in school or if you simply want a quick review, this Web site is great.