Bounce is a pretty cool app. It allows you to post notes to any webpage and share that page. This is done by taking a full-page screenshot of that page, which is a useful feature itself.
So if you want to get a full-page screenshot, just head over to BounceApp, type the url of the page and hit the Grab Screenshot button. No addons or additional software is required. And its free!
Take FullPage Screenshot of a WebPage