Publisher's description - Inline Search and Lookup 1.0.2
Simply hold the Alt key (but you can customize this option) and double click a word, or select it. The Inline Search and Lookup extension will open a small inline box with the results of your search.
The Inline Search and Lookup for Chrome extension works for any search engine or online dictionary, and allows you to specify your own. As of now, the default selection contains only a few choices, namely: Wikipedia, Answers.com, tfd.com and Google.
A little useful feature is the ability to specify default scrolling actions for each source, so when you perform a search, the pop-up will automatically scroll to the appropriate section of the page.