WordWeb is a free English dictionary and thesaurus application, and can be used to look up words in just one click. This is a very handy productivity tool and it’s really fast. I found their “for free use” licensing agreement quite amusing (though I fail on both accounts!):

You may use the program free of charge indefinitely only if
* You take at most 4 flights (2 return flights) in any 12 month period
* AND you do not own or regularly drive an SUV (sports utility vehicle).

I also like that it can be installed as a portable USB application (under Advanced Options during install).

Via Lifehacker