Some nice exercises!

http://www.cambridge.org/touchstonearcade

http://esl.about.com/library/quiz/bl_relative1.htm

http://www.testyourenglish.net/english-online/grammarquiz/relative.html

http://www.usingenglish.com/quizzes/234.html

http://www.oup.com/elt/global/products/headway/upp_intermediate/