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AnswerTips
AnswerTips® are small information bubbles that define any word when double-clicked. An AnswerTips-enabled site or blog means visitors get fast facts on 4 million topics provided by Answers.com when they double-click on any word, without opening a new browser or following outbound links. AnswerTips deliver instant definitions, explanations and facts including biographies, tech terms, geography, pop culture and much more. @http://www.answers.com/main/download_answers_win.jsp

Hold ALT and click on a word and a bubble with the definition will pop up.