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.
FlashBlock by josorek
No developer site listed
Description:
Sets up Chrome to default to blocking Flash content on webpages.
Pros:

Helps reduce download time and bandwidth usage since flash files aren't loaded by default.

Easy to adjust blocking filter to allow flash for entire sites via a button in the address bar.


Cons:

Sometimes removes stuff without an easy way to add it back with the extension running.