Grammar Usage and Practice   

Grammar Blast  Quizzes on the different types of speech

Guide to Grammar and Writing Excellent site teaches different parts of speech, then provides online quizzes to test your knowledge.  Also, go to the "Interactive quizzes" section, under "Ask Grammar, Quizzes..."  for more practice.

OWL: Handouts: Grammar, Punctuation, and Spelling  Online Writing Lab from Purdue University -- Another great site teaching parts of speech and providing quizzes