Most of us learned grammar during twelve years of school, but not all of those lessons have stuck with us! If you often find yourself questioning the proper use of commas and apostrophes, or you are wondering what the heck a split infinitive is, you might find these grammar references very helpful. There are tons of good grammar references and exercises available online, but I have done the work for you and found some of the best!
The Guide to Grammar and Writing, found at http://grammar. ccc.commnet.edu/grammar/, is an excellent reference that touches on tons of topics. Each topic includes a description of the proper usage, examples of correct and incorrect use, and some topics have exercises or quizzes.
The Purdue Online Writing Lab offers a grammar reference section at http://owl.english.purdue.edu/owl/section/1/5/. This is an invaluable resource for brushing up on grammar topics such as articles, prepositions, and apostrophes.
The Basic Elements of English: An Interactive Guide to Grammar offers an excellent interactive grammar reference at http://www.ucalgary.ca/UofC/eduweb/grammar/. This site is unique in that it provides instructions on proper grammar for every topic, and then allows you the opportunity to test yourself by typing answers or by clicking bubbles to fill in blanks.