For those using Mozilla/Firebird, do a search, and bookmark the result. Edit the bookmark, replacing the query with a %s. Set a keyword for the bookmark, and you can now type <keyword> <query> in the address bar to look up words in the dictionary. It's fantastic.
Here's the URL for webster, it's not as easy to get on your own as dictionary.com
http://www.webster.com/cgi-bin/dictionary?va=%s