Longman Dictionary of Contemporary English Online

A useful online reference tool from publishers Longmans: the search box and brief descriptions appear in a small pop-up box that can be kept open on the page without covering other pages you are using. Many of the words are illustrated, and entries commencing with the letters "D" & "S" allow the user to turn on a speaker and listen to alternative US and English pronunciations. Not as fully featured as the paid-for CD version.
www.ldoceonline.com/