CleverKeys is a free app for hot-key access to definitions at Dictionary.com, synonyms at Thesaurus.com and articles at Reference.com. Just select a word while writing in Word, for example, and look it up in your browser. As easy and much more comprehensive than MS Task Pane sources such as Encarta. Reference.com accesses Wikipedia and Columbia encyclopedia articles. A cool function of these sites is that results auto-generate citations in styles such as CMS and MLA, so you can cut/past source info into your notes.
Chicago Manual Style (CMS): marsupial. Dictionary.com. The American Heritage® Dictionary of the English Language, Fourth Edition. Houghton Mifflin Company, 2004. http://dictionary.reference.com/browse/marsupial (accessed: May 01, 2007).