Useful Chrome extensions for OS X

I know there is already a post out there for Safari extensions, 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, its pros/cons, and why you use it. - Link to the developer's website, rather than a direct download page. - Cite the application name like such: #appName by [developer] - Check to see if there are duplicates in this post by typing in the search box: `inquestion:this searchTerm` - Please do not give a price because that can localize your answer too much.