I know there is already a post out there for <a href="http://apple.stackexchange.com/questions/24196/useful-safari-extensions">Safari extensions,</a> but Chrome extensions are completely different. So, what extensions do you think are useful? Please read these simple rules before posting:
	

 - Include only one extension in each anwer.
 - Include a description of the extension and its pros/cons. 
 - Link to the developer's website, rather than a direct download page. 
 - Cite the application name like such:  
 #appName (price) by [developer]
 - Check to see if there are duplicates
   in this post by typing in the search box:   
`inquestion:this
   searchTerm`