Free online dictionaries, you might know a few, but there is a lot to explore.

General:

http://www.answers.com

http://dictionary.cambridge.org/

http://www.merriam-webster.com/

http://www.yourdictionary.com/

http://www.thefreedictionary.com/

Medical Dictionaries :

http://www.online-medical-dictionary.org/

Merriam-Webster Medical Dictionary

Legal :

http://dictionary.law.com/

Computing :

http://foldoc.org/

Business :

http://www.investopedia.com/dictionary/