http://www.macmillandictionary.com/
http://www.merriam-webster.com/
http://dictionary.reference.com/
http://dictionary.cambridge.org/
http://oxforddictionaries.com/
http://www.thefreedictionary.com
Little Explorers Picture Dictionary
http://www.enchantedlearning.com/Dictionary.html
http://englishwilleasy.com/ An Online Picture Dictionary