Dictionaries

http://dictionary.cambridge.org/

http://www.americancorpus.org/

http://www.lexipedia.com/

http://www.thefreedictionary.com

http://www.hyperdictionary.com

Other Vocabulary Resources

http://www.freerice.com/

Reference Desk: http://www.refdesk.com

Academic Word List Exercises

More Academic Word List Exercises

Idioms for Self Study