http://www.ego4u.com/en/cram-up/grammar/passive/exercises?simple-present
http://learningapps.org/12735
http://www.english-grammar-lessons.com/passive/exercise2.swf
http://www.ego4u.com/en/cram-up/grammar/passive/exercises?02
http://faculty.mdc.edu/kbiache/ferriswheels.htm
http://www.eslgamesplus.com/passive-voice-past-passive-esl-grammar-sentence-activity-fun-game-online/
http://www.eslgamesworld.com/members/games/grammar/basketball/active%20vs.%20passive%20voice/active%20vs.%20passive%20voice.html