For extra practice with articles:
From Purdue’s OWL: The Use and Non-Use of Articles – including countable vs. noncountable, first vs. subsequent mention, and general vs. specific. This page has a very good overview of everything about article use in English.
About.com: Definite and Indefinite Articles – including a short quiz.
Quiz on definite and indefinite article use
Three more quizzes about articles (graded by level)