The English Dictionary Lookup for Chrome adds a double-click to look up words on any webpage. You can even search for words inside the dialog box. All this is done completely without connecting to an external site, so it works offline.

So, how does this work? When you're inside a web page, double-clicking anywhere selects a word for you to lookup. The word definition will appear on the top of the screen with an "i" next to it and an "i" in the URL bar of your browser. If the definition is found in the dictionary, a dialog box will show up with the word definition. You can even lookup synonyms and other forms of the word.