Longman's Dictionary of Contemporary English Online
      www.ldoceonline.com

Heinle's Newbury Dictionary for American English
     http://nhd.heinle.com/home.aspx

Merriam Webster's
     http://www.learnersdictionary.com/

(also has a microphone feature that will pronounce the word for the student! Only some words are available with this feature but their standard dictionary--http://www.merriam-webster.com/-- has the feature for all words.)