Vocabgrabber is a pretty amazing online vocabulary tool from the folks at VisualThesaurus. The application grabs the vocabulary on given web page and displays it. You can click on each word and get information about that word. And, you can put a Vocabgrabber applet on your browser toolbar.
Does the above sound too techy? If so, go take a look at Vocabgrabber; it's pretty dang cool. This one's a game-changer.