Helpful Online Dictionaries

http://www.onelook.com

http://foldoc.org/ - Computing Dictionary

http://www.medterms.com/script/main/hp.asp - Medical Dictionary

http://www.thefreedictionary.com/

http://biology.about.com/od/prefixesandsuffixes/a/aa020106a.htm -- Biology Prefixes and Suffixes

http://www.dictionary.com  -- you can use wildcards with this one.  For instance to find all words that being with the prefix Quad, simply enter quad* and click on search.

http://www.wikipedia.org/  -- you can also use wildcards with this one. 

http://tryit.worldbookonline.com/