Take a full or partial snapshot of any website or even upload an image that you'd like to share or send to someone. You can draw arrows, lines, circles to point out things or even move, erase or delete selections and even annotate the snapshot by placing text right on the image.
Then you can copy the image to the clipboard and paste it into your email or download the image onto your desktop. You can also get a direct link to the image and share it on Facebook or Twitter.
The main advantage of this extension is it's like Snagit, but free, fast and easy to use and can be used to create mockups.