Answers.com, "the best definitons and explanations for over one million topics," offers 1-Click™ Answers software, which lets you Alt-click a word in any application you're running on your computer for a quick Answers.com lookup. (Ooohh, we always wondered about "pie in the sky.")
1-Click™ Answers is still in beta, available for Mac or PC, and from the looks of things, is spyware-free. If you're not down with installing a whole new application, there's Firefox extension which will add Answers.com to your Firefox search box.
1-Click™ Answers [Answers.com]