Articles: a, an, the.

Definite and indefinite articles: Rules and Fill in the gaps.
http://esl.about.com/library/beginner/blathe.htm
Articles Quiz http://www.englishmedialab.com/Quizzes/advanced/definite%20indefinite%20article.htm
Fill in the gaps with articles
http://grammar.ccc.commnet.edu/grammar/quizzes/cross/articles_quiz.htm
fill in the gaps with articles http://grammar.ccc.commnet.edu/grammar/quizzes/articles2.htm
Articles worksheet
http://www.esl-lounge.com/level2/lev2articlewsheet.shtml
Articles – Quiz
http://esl.about.com/od/grammarintermediate/a/a_articleq1.htm