Wikilook 1.3.1 -- Firefox Add-On

Wikilook 1.3.1 is a Firefox add-on that allows you to look up the meaning of a word on a web page without leaving the current page. Simply hold down shift, then hover the mouse pointer over the word whose definition you want, wait a second or two. A box will appear with the definition of the word, a usage example, and a direct link to the word in Wikipedia.