Useful Chrome extensions for OS X

Question

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.