Test Your Grammar & Punctuation

Great, fun, and free: the Blue Book of Grammar and Punctuation's collection of free online quizzes. It's addictive. (They also offer a free reference guide to many grammar rules - it's worth bookmarking.)

Another great free site, with lots and lots of quizzes: About.Com's Learning English as a Second Language Quiz Collection.

Finally, if you really love this stuff, check out: UsingEnglish.Com's selection of over 250 free quizzes; and the SAT preparation quizzes for Basic Grammar, Intermediate Grammar, and Advanced Grammar.