Did you know that you can quickly look up words in the New Oxford American Dictionary, Oxford American Writer's Thesaurus, Wikipedia, or Google right from within InDesign? If you use a Mac running OS X 10.5, you can!
Just select any word in your InDesign layout, and choose InDesign > Services > Look Up in Dictionary. The Apple Dictionary application will be launched, and the selected word will be looked up in the dictionary automatically. If you select a phrase instead of a word, it will be looked up in Wikipedia through the Dictionary application.
To search for a selected word or phrase in Google, choose InDesign > Services > Search With Google.
This isn't really an InDesign feature at all. The options in the Services section of the InDesign menu (aka the Application menu) are provided by OS X, and work in many Macintosh applications (but not Illustrator or Photoshop, unfortunately). So this is a Macintosh-only feature. I don't know of an equivalent cross-application technology available on Windows.