Someone had recommended the one at dic.naver.com a while back. I love it; works better for me than google or even the multilanguage package on Microsoft Office. You can choose from any number of dictionaries (English, Japanese, Chinese to Korean and vice versa, hanja dictionary, etc.).
The caveat is that all the menus are in Korean, but that's an excellent opportunity to learn a few useful words
. The English-Korean (영어사전) direct link is http://endic.naver.com/